TANSTAAFL is an acronym for the adage "There Ain't No Such Thing As A Free Lunch," which was invented by
science fiction writer Robert A. Heinlein and promulgated in his
1966 novel
The Moon is a Harsh Mistress. Simply put, it means that you can't get something for nothing. Even if something appears to be free, there's always a catch -- someone wants something from you in return.
This argument may also be applied to natural physical processes; see thermodynamics.