What is a Z chord on ukulele?

A z chord is not a real thing. It is a method of making muted strings or, er, clicky noises.

Z chord can be useful for 5 reasons:

1. you’re playing in a group and you’ve forgotten the chords
2. you’re playing in a group and you want to highlight someone’s solo
3. you want to make a percussive sound, either in a group or alone
4. you want to mute the end of a note or chord
5. you want to practice a set ‘strumming pattern’ and nail it, more on that, here