The octave in which a note is notated and/or played is indicated by a number after the note name.
A4 is the middle A on the piano (A4=440Hz) and the usual tuning tone.
To indicate that all notes should be played an octave higher than notation, use ottava (8va)
|A0 is lowest note on piano, lower is sometimes found on a large pipe organ
|Great because it used to be written with a capital letter
|Small because it used to be written with a small letter
|C4 is middle C on piano as well as in notation on a piano system. Middle A is also in this octave
|C6 is the 'high c' of a soprano voice
|C8 is the highest note on a piano, much higher notes are not musically interesting (because inaudible for many people)