120 BPM means 120 beats every minute: one beat every 0.5 seconds (500 ms), or 2 beats a second. In classical terms it is Allegro, fast and bright (120–155 BPM).
You often hear tempos around 120 BPM in dance pop, disco, house and marches. The metronome above is already set to 120 BPM in 4/4: press Start, and change the time signature, accents, subdivisions or sound if your song needs it.