All of the Thomist arguments are variations on a common theme, Thomas Aquinas was looking for a "first cause" for the universe, but not in a time related sense. He was looking for a "First Cause" in terms of rank. What this means is that the thing that causes must be"big enough" to adaquetely explain the resulting thing that is caused. If the thing that is caused is everything that exists, then it's cause must truly be substantial. In the case of the Thomist arguments, that thing which causes everything else must be so big, so all encompassing, that it transcends the status of "possible" and thus must have the status of a "necessary" thing.
Everything you see around you is "possible," but not "necessary." Is the world such that the specific chair you are sitting in, or even you yourself, HAVE to exist? Certainly both you and the chair are "possible" since you and it exist, but could the world have been otherwise such that you or that chair did not exist? Of course it could. That is what is meant by "possible," but not "necessary." Is there anything that is absolutely necessary, such that there could be no conceivable world where that thing did not exist? That is the definition of a "necessary" thing, or being, and the question that Thomas Aquinas was trying to answer.
(1) We see in the world things that exist but do not have to exist, that is to say, their existence is not necessary but merely possible, for we see them coming into and out of being.(2) All things cannot be merely possible things, because:(a) If a thing is merely possible, then at some time it did not exist.(b) And if all things were merely possible, then at some time all things did not exist: there was nothing.(c) But if at one time nothing existed, then nothing would exist now because something that does not exist cannot bring itself into existence.(d) But this contradicts observation.(e) Therefore, all things cannot be merely possible things; there must be something that is necessary.
I have to add a brief comment about point (b). The argument rests on this point, and this will probably be the area where the natural materialist picks his fight. However, because of the nearly universal acceptance of the "Big Bang," natural materialists will not be able to make a strong case that there was a moment in the past when nothing existed. While there have been a few attempts to incorporate the "Big Bang" into theories of an eternal universe (these theories paint a picture of a universe expanding and then reversing to collapse on itself, which in turn causes another "Big Bang," which is followed by another outward expansion, repeated endlessly into eternity) such theories have mostly received little attention and cannot be confirmed, verified, or even refuted in any way, due to our inability to see beyond the Planck Boundary. Such theories with so little to offer scientifically will never rise to a level of importance beyond "sideshow curiosity," and are thus unimportant.
That said, I think the rest of the argument is beyond reproach.
So, if the argument is true then there is such a "necessary" thing. What does that mean? It means that whatever this "necessary" thing is, it is responsible for everything else that exists. Most of us would call such a necessary thing, or being, God.