In more detail
Seconds per beat are 60 / BPM, so 120 BPM is a beat every half second. The time signature says how beats group into bars: in 4/4 there are four beats to a bar, so 120 BPM is 30 bars a minute. A metronome keeps that click steady while you practise; common tempos run from about 60 (slow) to 180 (fast), and Italian markings such as adagio (slow), andante (walking pace) and allegro (fast) describe loose ranges rather than exact numbers.
Why it matters
Delay and echo effects, loop lengths and video cuts are timed to the beat, so producers convert BPM to milliseconds. DJs match the BPM of two tracks to mix them. Practising a hard passage slowly with a metronome and raising the BPM in small steps is a standard way to learn it cleanly.
Example
120 BPM: 60 / 120 = 0.5 s per beat
Quarter note = 500 ms, eighth note = 250 ms, dotted eighth = 375 ms
32 bars of 4/4 = 128 beats = 128 × 0.5 s = 64 s
Common confusion
Heart rate is also counted in beats per minute, but there it is a measurement, while in music BPM is a tempo you choose. Dividing BPM by 60 gives the beat frequency in Hertz (Hz): 120 BPM is 2 Hz.