7/8 has seven beats in each bar, and each beat is an eighth note (that is what the lower number means). In practice it is felt as three groups of uneven length (2+2+3). You will hear it in Balkan music, progressive rock and metal. Rupak taal, with seven matras grouped 3+2+2, is a close Indian cousin.
The metronome above is set to 7/8 with the beats grouped 2+2+3: an accent on beat 1 and a medium accent at the start of each group. The tempo counts eighth notes, so 210 BPM is 210 eighth notes a minute. Tap any beat to change its accent.