a perfect octave (or octave for that matter) is when you play 2 notes that are the same in pitch but one is 8 tones lower (or higher) then the other.
ex;
Play the note C on the 5th str. 3rd fret, then play C on the 2nd str 1st fret...this is an octave.
The 1st and 6th strings (both E) are seperated by 2 octaves.