Welcome, @digbit!
BPMs are measure of how “fast” a song is supposed to be played. The beats in the name usually refer to quarter notes (unless otherwise specified). Thus, 100 BPM means that per minute 100 quarter notes should be played. In that sense, 120 BPM is faster (more quarter notes per minute) than 100 BPM, while 80 BPM is slower.
As a reference: 60 BPM is one quarter note per second, 60 per minute - rather a moderate tempo.
Metronomes can be set to a certain BPM value to give you the speed of a song.
I hope this is what you were wondering about!? 