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 (e.g. 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. An example of this would be the notes F and G on the sixth string or the first string, or the notes G (open) and A (2nd fret) on the third string. Check these on the diagram below and on your guitar.