The meaning refers to the fact that there is nothing for free, everything has a more or less evident cost.

The is no such a thing as a free lunch:

• 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 nothing. The phrase is also known by the acronym of 'there ain't no such thing as a free lunch' - tanstaafl.

Origin

• Before discussing the origin of 'there's no such thing as a free lunch' it would be useful to go back to the days in which lunches were free. Free lunch was a commonplace term in the USA and, to a lesser extent in Britain, from the mid 19th century onward. It wasn't used to describe handouts of food to the poor and hungry though, it denoted the free food that American saloon keepers used to attract drinkers; for example, this advertisement for a Milwaukee saloon, in The Commercial Advertiser, June 1850:

At The Crescent...

Can be found the choicest of Segars, Wines and Liquors...

N. B. - A free lunch every day at 11 o'clock will be served up.

Free lunches,

• often cold food but sometimes quite elaborate affairs, were provided for anyone who bought drink....... Indeed, some saloon keepers were prosecuted for false advertising of free lunch as customers couldn't partake of it without first paying money to the saloon.

The concept was elaborated later into an important economic theory.

• It was into this context that the economic theorists enter the fray and 'there's no such thing as a free lunch' is coined. It isn't known who coined the phrase. It certainly wasn't the economist Milton Friedman, who was much associated with the term. He was a celebrated Nobel Prize-winning economist and his monetarist theories were highly influential on the Reagan and Thatcher administrations in the 1980s and 90s. Friedman certainly believed that 'there's no such thing as a free lunch' and he published a book with that title in 1975, but wasn't, and never claimed to be, the originator of the phrase.

• The phrase appears to have come about in response to the libertarian views of Henry Wallace, the US Vice President between 1941 and 1945.

(The Phrase Finder)