In both classical music and jazz, we often encounter the concept of voice leading. What is it? What does it do?
The term "voice leading" comes from choral music, but it applies to chord progressions in all sorts of arrangements for all sorts of instruments. A chord progression has two dimensions: horizontal and vertical. Let's say you are playing four-note chords in sequence. You can think of the progression as a series of block chords (vertical), but you can also think of it as four independent musical instruments (horizontal), which play one note at a time, playing four different melodies which line up to make the chords. So voice leading is the process of writing four melodies which are smooth and logical and easy to sing or play, in the service of creating the chord progression.
For example, if you were to compose a melody with a certain chord progression, and arrange three singing voices beneath the melody to flesh out the chords, you would usually not want the melodic lines of the three singing voices to have to jump around across awkward wide intervals from one note to the next. So you would try to follow the rules of voice leading outlined in classical music theory, to produce smooth, musical lines.
Here is a rough example I have created from a Bach chorale.
You might think of this piano part as a series of chords:
However, it is also four melodies sung simultaneously, using voice leading:
Orchestral composers think about voice leading. Pianists who improvise in jazz pay attention to voice leading, which you can think of as how to play a chord progression with the least amount of changes in hand position and the smoothest fingering patterns.
Unfortunately, due to the nature of the relationship of the pitches on the six strings of the guitar, the concept of voice leading is very hard to implement on guitar, and most guitarists don't get around to working with the concept. Guitarists, in contrast to piano players, tend to think of chords as certain block shapes that they memorize, and they give little thought as to creating different inversions and voicings of chords that would create a smoother voice leading in a chord progression.
As Wheat says, voice leading mostly doesn't happen on guitars, but there are a few cases where it does:
using certain patterns, like Travis-picking, the bass-line may be a more or less distinct part throughout the piece.
Every instrument that can make a pitch has a voice. Voice Leading is the study of how these voices move, resolve, and fit with the harmony. Every melody, every chord, every harmony can be looked at from a voice leading perspective. How the voice moves, what the range of the voice is, it's function in the harmony, how the voice interacts with the voice are all part of the study. Classical voice leading is different than Jazz voice leading and so are the rules that govern them. Since Jazz evolved from the classical, I would pick up a music theory book and study classical four part counterpoint and then move on to Jazz. There are many books to this topic and many do and don't that differ from style to style way more than can be written here.