Very, very big, especially when talking about things in space like black holes.
Supermassive is a word used to talk about really, really big things, like the biggest things in space.
Examples:
The supermassive black hole is at the center of the galaxy.
A supermassive star can be much bigger than our sun.
The supermassive galaxy contains billions of stars.
I saw a supermassive sandwich at the store!
The supermassive elephant was as big as a house!
The supermassive mountain was so big, it touched the sky.