As mentioned, a certain kind of note will be “assigned” two important things in, basically, any piece of music:
- A number of counts (from 1 to more than 1 or a fraction of 1 — but usually 1)
- An absolute duration, determined by the tempo of the piece.
- Ex: If you see q = 60 bpm (“quarter-note equals 60 beats per minute”), then you know that each quarter-note is supposed to last for 1 second. (In other words, there are 60 of them in each minute.)