9/8 has nine 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 beats, each split into three. You will hear it in slip jigs and some ballads (three groups of three). The Turkish and Balkan 9/8 groups the beats 2+2+2+3 instead.
The metronome above is set to 9/8 with the beats grouped 3+3+3: an accent on beat 1 and a medium accent at the start of each group. The tempo counts the dotted-quarter beat you feel (three per bar); every eighth note still clicks, with accents at the start of each group of three. Tap any beat to change its accent.