Cheese is a dairy product that's made from milk solids. Many delicious foods are made even more delicious with melted cheese, including pizza and nachos.
Cheese is made from the milk of cows, goats, sheep, or buffalo, and it's acidified before the solids are separated and pressed. There are many kinds of cheese, ranging from soft, spreadable cream cheese to hard parmesan. An old fashioned meaning of cheese is "stop" — you might say, "Cheese it!" This comes from 1800s thieves' slang, while the instruction a photographer gives, "Say cheese!" dates from the early 20th century.