In Music Theory, An Interval is the distance between two notes. For example, if you play the note Middle C (located near the center of the piano) and then play the note High C (located 8 notes to the right of the piano; also to the left of a group of 2 black notes), this is called an interval of 8 notes; usually written simply as an Octave.

*Diagram of an interval of an Octave (8th)

Other intervals include a second (2nd) from a C two a D:

*Diagram showing an interval of a 2nd.