A pithy way of expressing the idea that the shape of a structure is determined at least in part by the function it performs. This is a useful idea to keep in mind when trying to understand why something (often a plant or animal) looks the way it does, or when you're designing something yourself.
If x is going to do y, then x had better be built in a way that allows y. This is a big idea in biology, where there is no reason for things to be all fancied up (except for functional reasons like sexual selection). As far as human-made devices go, a product's main function is usually to be sold, whatever the advertisements might say. You see some weird forms out there.
This statement can be biologically misleading because with evolution, it is also the case that function follows form.