The word 'Octave' comes from the Latin word 'Octavus' meaning 'eighth'.
In music an octave is the distance from one note to the next note of the same alphabetical name.? From one C to the next C, or from one G to the next G.? These notes are at an 8 note interval up on your fret board.? When counting, we ignore the sharp or flat notes.? So count only whole notes and include the first and last notes in your count.
Here is what it looks like in music.

ONE OCTAVE
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This example illustrates the two C notes but it applies just the same to any of the notes you have learned.
Playing an Octave
In written music, when two or more notes are to be played together, they are written directly in line with each other, as illustrated.

When you find notes joined together like this, they must be played together as an octave and not one after the other but you don't have to play octaves.? You will find it gets easier as your ability increases but the roof wont fall in if you leave octaves out altogether.??I am not even going to explain how to play these two notes together at this point - that comes later - but for now just be aware of what it is.