An octave is the range of 8 notes of a major scale. The first note and last note of a major scale always have the same name. In the C major scale the distance from the lowest C to the C note above it is one octave (8 notes).
The following example ascends and descends the C major scale.
Each of the 8 notes in the major scale is given a scale number.
The distance between two notes is called an interval.
In any major scale the interval between the 3rd to 4th note and the 7th to 8th note in the scale is one semitone (1 fret) apart. All other notes are one tone (2 frets) apart.