A Note is a frequency. We assign notes letters from A-G (igonoring sharps and flats.) For example The frequency 440 is Concert A (A4), the note they play in the orchestra when they tune up before they start.
They were already tuned when they came out but they do it anyways.
If you double the frequency of a note your brain will connect the two notes as the same but up some distance. We call distance octave. They call it an octave because there are eight notes in the major scale, but we don't know about that yet in this bish.