When someone mucks something, they make it dirty or messy. Mucks are also things like dirt or mud.
If something is mucked, it is usually not good because it is dirty or messy.
Examples:
She mucks up her clothes when she plays outside.
The little kids love to play in the mucks.
He mucks the floor with his dirty shoes.
The dog mucks up the yard when it rains.
The mucks from the garden got all over the kitchen floor.
The mucks on the road made it hard to walk.