What does this sentence mean? ?

He didn't have to do anything directly to make the dinner come into existence.  It just happened.  Usually, though, the phrase is somewhat hyperbolic, an exaggeration to make the point, and the point would be that all the difficult parts about getting dinner were done before he started doing anything.  All he had to do were the trivial things, like pick up the food and put it on a plate.

A typical case would involve someone who had no food so was going to have to do a lot of work to even get food (find it, gather it, perhaps kill it, bring it back home), when, suddenly or randomly, the food presented itself and all he had to do was harvest it, more or less a case of just pick it up and start eating.  If the idea were the appearance of an edible animal that all he had to do was kill and cook, the dinner really did not take care of itself, but it sure did cut out a lot of the work.  What was going to be eaten was decided for him.