50 BPM means 50 beats every minute: one beat every 1.2 seconds (1200 ms), or 0.83 beats a second. In classical terms it is Largo, broad and slow (50–59 BPM).
You often hear tempos around 50 BPM in very slow ballads, ambient music, and slow 12/8 blues counted in dotted beats. The metronome above is already set to 50 BPM in 4/4: press Start, and change the time signature, accents, subdivisions or sound if your song needs it.