The meaning of this in economic terms is that scarcity exists and therefore everything has an opportunity cost. Since everything has an opportunity cost, nothing is ever free.
If I am given a lunch for "free" there is still a cost to that lunch. The chicken that I ate for my lunch could not be used in some other way. The feed that the chicken ate while it was alive could not be eaten by anyone or anything else. The resources and labor that went into making the chicken feed and raising the chicken and butchering it and transporting it and cooking it all could have been used to do something else. All of those "something elses" will never happen since I have gotten that "free" lunch.
Thus, my lunch was not free because of the opportunity costs of making the lunch.