75 BPM means 75 beats every minute: one beat every 0.8 seconds (800 ms), or 1.25 beats a second. In classical terms it is Adagio, slow and expressive (66–75 BPM).
You often hear tempos around 75 BPM in ballads, reggae one drop grooves, slow romantic film songs and lo-fi. The metronome above is already set to 75 BPM in 4/4: press Start, and change the time signature, accents, subdivisions or sound if your song needs it.