If something is stained, it has a mark or color that is hard to clean off.
Stained things can make people feel like something is dirty or old.
Examples:
The shirt is stained with ketchup.
She stained the wood with a dark color.
The carpet is stained with juice.
I don't want to sit on the stained chair!
The whole town was stained red with paint.
He stained the floor and now it's all different colors.