What do economists mean by the saying, "There is no such thing as a free lunch?

What do economists mean by the saying, "There is no such thing as a free lunch?" How does it apply to ads such as, "Buy 3 tires and get the 4th free", or "All fans in attendance get a free Padres cap"?

What do economists mean by the saying, "There is no such thing as a free lunch?" How does it apply to ads such as, "Buy 3 tires and get the 4th free", or "All fans in...