A tub is a wide, open container. You might have a tub of butter in your fridge, but if your mom tells you to hop in the tub, stay out of the kitchen. She means it's time for a bath.
Tub is an informal word for bathtub, the fixture in your bathroom that can be filled with water and bathed in. Other tubs are containers used to hold liquid or other materials — think of a tub of margarine or a tub of ice cream. In the 17th century, tub was slang for "pulpit" or "lectern," and a tub thumper was an overly enthusiastic orator or preacher.