If something is rustier, it has more rust on it.
Rustier things are usually old and not very good.
Examples:
The old bike is rustier than the new one.
My car is getting rustier every year.
The rustier the key, the harder it is to turn.
I don't want to sit on that rustier chair!
The rustier the treasure, the more valuable it is to some people.
The rustier the sword, the more dangerous it is to touch.