Chromatic notes on a guitar (or any instrument) are a half-step/semitone apart, e. g. , the chromatic notes on the scale of C are C, C#, D, D#, and so on.
The chromatic scale applies to all instruments, and can be applied to the guitar. The chromatic scale can start anywhere in this list, repeat and go either direction, C C# D Eb E F F# G G# A Bb B. As long they are within 1-half step it is chromatic.
Chromatic notes are separated by one half-step. On a guitar, one half-step is equal to one fret on the fingerboard, so chromatic notes are one fret apart.
Same as on any instrument - consecutive notes a semitone apart. Many jazz guitarists play chromatic solos - basically, they mostly play on a pentatonic scale for the significant notes in the solo, and finger all the notes in between as "passing notes" to link those "long notes" together. Sometimes it is "technique over talent" - sometimes it's real talent.
