We see that things that lack knowledge, such as natural bodies, act for an end, and this is evident from their acting always, or nearly always, in the same way, so as to obtain the best result. Hence it is plain that they achieve their end, not fortuitously, but designedly. Now whatever lacks knowledge cannot move towards an end, unless it be directed by some being endowed with knowledge and intelligence; as the arrow is directed by the archer. Therefore, some intelligent being exists by whom all natural things are directed to their end; and this being we call God.
This one at first glance does not seem meaningful. Is it true, and obviously true, that things in nature act for an end? What would be the “best result” for a rock toppling from a hill? To avoid shattering into smaller pieces? And do rocks so falling do anything to avoid this?
But I think what St. Thomas is getting at in saying natural bodies act for an end is the idea we now call natural law: that natural bodies act in a broadly predictable way, roughly the same way every time. Drop a stone, and it will fall down, not up or sideways. Heat a solid, and it will turn liquid. This is, of course, what science is all about.
The fact that we can predict what will happen—better and better as our science improves—implies, means, that natural processes have an end, a goal.
Hence, they are directed by some intelligent being, since inanimate objects necessarily cannot choose their own goal or behaviour.
Conversely—so as not to assume the antecedent—if there were no goal to nature, one would expect the behaviour of natural objects to be random. This time the stone would fall down; the next time it would fall up, or stay in place. The solid, heated, would grow more solid, next time disappear, next time turn into a butterfly.