A useful way to measure pitch is to work out how far apart two notes are in terms of their pitch.
We already know about octaves as one measure of pitch, but what about for smaller steps up and down in pitch.
To measure these distances we can talk about tones and semitones. On a keyboard instrument like a piano or organ each of the keys is one semitone apart. The distance between C and C#, for example, is one semitone.
A tone is simply a distance in pitch of two tones. F to G, for example is a distance of a tone.