5/8 has five beats in each bar, and each beat is an eighth note (that is what the lower number means). In practice it is felt as two groups of uneven length (3+2). You will hear it in Balkan and Greek folk dances and progressive rock, felt as a long and a short beat (3+2).
The metronome above is set to 5/8 with the beats grouped 3+2: an accent on beat 1 and a medium accent at the start of each group. The tempo counts eighth notes, so 180 BPM is 180 eighth notes a minute. Tap any beat to change its accent.