115 BPM means 115 beats every minute: one beat every 0.52 seconds (522 ms), or 1.92 beats a second. In classical terms it is Moderato, at a moderate speed (108–119 BPM).
You often hear tempos around 115 BPM in pop, pop rock, Latin and soul. The metronome above is already set to 115 BPM in 4/4: press Start, and change the time signature, accents, subdivisions or sound if your song needs it.