A semitone is the smallest distance between two notes used in western music. On the guitar, notes which are a semitone apart are one fret apart. For example, the note C on the 3rd fret 5th string is one semitone above the note B at the second fret. This could also be reversed, i.e., the note B is one semitone (one fret) below the note C.
Notes which are a tone (two semitones) apart, are two frets apart. For example, F and G on the sixth string or the first string, and the notes G (open) and A (2nd fret) on the third string.