What does the idea in economics that “there is no free lunch” mean?

The economic theory, and also the lay opinion, that whatever goods and services are provided, they must be paid for by someone - that is, you don't get something for anything. The phrase is also known by the acronym of 'there ain't no such thing as a free lunch' - tanstaafl. One of the most famous quotes in history is, "There's no such thing as a free lunch." The adage refers to the idea that it is impossible for a person to get something for nothing.

Every choice you make has a next-best alternative that you could have chosen but didn't. That foregone opportunity is known as opportunity cost. That is, the price you paid for doing whatever it is you did was the opportunity you can no longer enjoy.