Think of the octaves in relationship to the piano. You have middle C and when you go to the left the next C is a lower octave, but a C nonetheless. Same as when you find the next C to the right, it will be a higher C. You will hear some refer to a singer that can sing the full range of C to C, sometimes two ranges (that would probably make them an opera singer or a bass singer..lol)
On the guitar the open 6th string is a low E, but the 1st string E open is a a higher E. When you run the scale out of a Key such as E your next octave of E will be your eighth note. Do, re, me, fa, so, la, ti, do. Same in all keys.
I hope I helped a little.