When you buy something, you pay money in order to own or obtain it. You might buy your mom a big bouquet of flowers for her birthday.
When you buy furniture, food, a car, or candy, you purchase it, exchanging a certain amount of money for it, and when you call something "a buy" or "a steal," it's a real bargain. When you say, "She said she won the lottery, but I don't buy it," you mean that you don't accept that fact as the truth. Even more colloquially, to buy it can also mean "to be killed."