The basics
Pitch, Frequency & Octave
When we described a note as being high or low it is the 'pitch' of the note we are referring to. Pitch works in exactly the same way as frequency, which is the rate of vibration of the material making the sound (in this case the guitar string).
Take a look at these two different C notes as they appear on the guitar:
You can see that doubling the frequency produces the same note but in a higher register. It is easy to see this on the guitar.
If you play your open E string 1 then play the note half way along the string (at fret 12) the note at fret 12 is another E. Halving the length of the string doubles the frequency:
In music, the distance between two sounds is called the interval. The interval between the two C's or the two E's is called an octave. In terms of frequency, doubling the frequency of any note increases the pitch by an octave.