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It is used to tell someone to be careful how much money they spend, because money must be earned and it does not come freely.

Son: Parents, I need a new car.

Father: Pardon me, we bought you a car last year. Money doesn’t grow on trees. Or you can find a job and buy it yourself.

The idiom relates to the fact that trees produce fruits for people to enjoy regardless they are cultived or not by the people while you must work to earn your money. It appeared in print in the 1800s.