I've just purchased a bugle which is tuned to B flat. What does it mean for a brass instrument to be tuned to a particular note and how does this determine the notes that can be played on the instrument?
Your bugle, being a simple length of tubing with no valves or keys, can play the first half-dozen or so notes of the harmonic series. That's the basic note (fundamental), the octave above, then the fifth above, the next octave, the third and fifth above that... You know, the notes of a bugle call.
If you play from notation, these notes will be written starting on C. C, C, G, C, E, G... As it's a 'B♭ Bugle' the actual sounding notes will be B♭, B♭, F, B♭, D, F...
(You CAN just about play the fundamental, but it's a bit of a fart. Mostly you'll start on the next harmonic up, written as Middle C.)
Transposing instruments sound at a different pitch from the written notes they play. There are various reasons why this method developed hundreds of years ago. Today, tradition means that we still use these transpositions. Your bugle, being in Bb means that if you play a C, the note that you produce will be a Bb.
The B♭ bit means that the notes it produces when you blow properly through it all belong to the harmonic series of B♭.
It's the most common key for bugles - and a lot of other brass instruments. It's purely down to the total length of tubing used for the instrument - from mouthpiece to end of bell. As you tighten the embouchure, the notes get progressively higher, making the harmonic series of base note, octave, P5, 2nd octave, M3 and so on.
Those notes are probably familiar in different orders as bugle calls used in the military etc. - Reveille, etc. There are several notes from the whole major scale that are unobtainable, so we end up with so-called bugle calls.
A brief note (sic) on reading music - for your bugle, because it plays 'in B♭', any dots will need to be written out a whole tone higher than the music for others playing on non-transposing instruments, in order that you sound in tune with them.
Playing on a Bb instrument like a flugel means that each note is transposed a wholetone (major secound) lower.
You play the scale of C major but the result will be the Bb-major scale.
Or a piece in F for Bb-Trumpet or flugel is written in G major for the brass instrument but the piano accompaniment. So the Bb instrument always have 2 sharps more or 2 flats less than the C- instruments.
(There are some musicians like trombonists who are used to read in the absolute notation and not the transposed. But they are also able to switch between the 2 notations and also the treble and bass clef. The notation for tenor instruments can be in bass clef or treble clef. If it's in treble clef it is usually notated 9 tones higher as the Bb-instruments is transposing an octava and 2nd down.)