90 BPM means 90 beats every minute: one beat every 0.67 seconds (667 ms), or 1.5 beats a second. In classical terms it is Andante, at a walking pace (76–107 BPM).
You often hear tempos around 90 BPM in hip-hop, R&B, funk and many film songs. The metronome above is already set to 90 BPM in 4/4: press Start, and change the time signature, accents, subdivisions or sound if your song needs it.