I've heard that guide tones are very useful while improvising, and seems to be an important concept for jazz theory.
What is a guide tone? How can it help improvisation?
Guide tones in a lot of ways are what "define" the feel of a jazz chord, and get you from one chord to the next. This should make it pretty obvious why they are useful in improvisation, since anyone who's ever tried and failed to improvise over an unfamiliar set of changes before didn't know what they were supposed to sound like and couldn't figure out where they were going. This explanation is best made restricting ourselves to 7th chords--effectively the basis of jazz harmony.
7th chords are said to have two guide tones: the 3rd and the 7th. These define the quality of the chord because the root and the 5th make up a stable harmony and define the root of the chord -- but based on how the guide tones interact with one another you'll see why the root is much less useful to keep track of than the guide tones.
Basically, take three C chords: Cmaj7, C7, and Cm7. The only differences between those chords are the 3rds and 7ths -- they always contain C and G. Thus, the 3rd and 7th define the quality of each chord (relative to one another).
So the other extremely interesting thing about guide tones is that generally (i.e. for 90% of chord changes), any two guide tones from one chord will be a half step or whole step away from two guide tones in the next chord in the progression. So by keeping track of them and where they are going, you kill two birds with one stone: hearing the quality of the chord, and how to get from point A to point B.
A simple illustration is a 12-bar blues. For example in C:
|: C7- F7- C7- - - | | F7- - - C7- - - | | G7- F7- C7- G7- :|
Now let's look at the guide tones for each chord; the 3rds and 7ths:
C7: E, Bb - 3rd, 7th F7: Eb, A - 7th, 3rd, half steps C7: E, Bb - 3rd, 7th, half steps G7: F, B - 7th, 3rd, half steps F7: Eb, A - 7th, 3rd, whole steps C7: E, Bb - 3rd, 7th, half steps
Obviously the trick here is that the fact that one guide tone can switch from the 3rd to the 7th from chord to chord means that the root of the chord can move by much larger intervals.
This is always going to be easier in less-harmonically-intense music, but the concepts still apply with more complex harmonies: find the money notes in each chord, and keep track of how they change from chord to chord. Coltrane was the master at this, and his ability to keep a complete set of pitches in his head and just move individual ones up or down from chord to chord is what gives rise to his "sheets of sound" approach to soloing, and harmonic monsters of charts like Giant Steps.
Guide Tones are a set of notes that outline voices in a chord or progression, usually spelling out what type of chord and descending in a linear fashion within the progression. The most commonly referred to Guide Tones are the 3 and 7 of a chord (often called The Guide Tones). Within most sub-genres of jazz it is standard to play chords with a 7 (major 7, minor 7 , dominant 7, etc.). Since a natural/perfect 5 appears in the most common chord types, such as -7, maj7, dom7 (and for mathematical reasons), it does not tell you much about the chord. The root of a chord is pretty essential to expressing the chord itself but not the most important thing for functional purposes. The 3 and 7 of a chord will tell you the most about what type of chord it is, hence being the most common Guide Tones. Note that the root note is usually played by the Bass player (or often left hand of the keyboard).
Circle of fifths progressions are the root of functional Jazz harmony. The II-V-I progression is a circle of fifths expression of the IV-V-I progression of Classical music. II replaces IV in the same fashion as a relative minor and creates the circle of fifths motion. VI-II-V-I is also very common in Jazz and is yet another extension of this motion. Using the standard Guide Tones, 3 and 7, outlining a circle of fifths chord progression, each of the two voices will either stay the same or move down by step. Also, the voice playing 3 on the first chord will then be playing 7 on the next chord and vise versa. For example, the 3 and 7 of each chord of a VI-II-V-I in C major:
Chord : 3 , 7
You can see a smooth linear motion between two voices:
Other notes can be referred to as Guide Tones as well and perform a similar back and forth between two voices with common tones and motion by step. A common example is the 9 and 5. Notice that neither of these tones bears the same importance of chord quality.
Chord : 9 , 5
Again, you can see a smooth linear motion between two voices:
The primary purpose of Guide Tones is to express the chord type. This is largely why they are so important for improvising. Being aware of the Guide Tones of each chord within the progression allows the improvisor to know what chord is being played, and with that, where they are in the form. Teachers often have students of all instruments practice singing the 3/7 Guide Tone lines to internalize them. While comping it is important to play the Guide Tones because they spell out the quality of the chord and that is what the soloing musician is basing their approach on, even if they are playing outside it is based on it being outside the changes they are expecting. Depending on what type of group you are comping for, you will be more or less encouraged to follow the soloist outside or to introduce harmonic substitutions not directly inspired by the soloist.
As is true with all music theory, these 'rules' are merely an expression of how to live out a specific style or genre of music and are ultimately more an explanation than actual rules.
If I'm understanding your question correctly, a guide tone is simply a note (usually a non-chord tone) that is used to connect certain chord tones in a phrase typically in a constant motion. This tone could be for example in a ii-V-I progression in C major: using an F# chromatically to connect an F in the ii chord (d minor) to a G in the V chord (G major). Of course these could also be chord tones like using a B as a transition from the V to the I.
I cannot stress enough the importance of these techniques in improvisation, as they make writing melodies and themes easier, as well as varying themes because you don't need to think as much about each specific note instead of the motion that the pattern created. They also help to smooth out phrases which a lot of intermediate-advanced improvisers have trouble with (myself included).