When you've really got to go, you need the toilet. It's what you use to relieve yourself after you've drunk way too much lemonade.
A toilet is the most basic feature of a modern bathroom. You can also use this word for the room that houses it, as in "Excuse me, where is the toilet?" — although in the U.S. most people say "bathroom" or "restroom" instead. Another meaning of toilet is "the routine of washing and dressing," and this old-fashioned definition is the original one, from a word meaning "cover or bag for clothes."