What is a moonquake?

MOONQUAKES ARE LIKE...

They're like earthquakes, but for the moon!

Imagine the ground shaking, but it's not just the ground. It's the very fabric of space-time itself.

They're like, really bad for the moon's fashion sense, though.

I mean, who thought it was a good idea to put a moonquake-proof fashion trend on a planet with a 4 billion year old crater record?

Why do we care? What is a moonquake?