He explains the meaning of the word.
Let us just look at the first four sentences together how they are written in German:
1. Die Welt ist alles, was der Fall ist.
2. Was der Fall ist, die Tatsache, ist das Bestehen von Sachverhalten.
3. Das logische Bild der Tatsachen ist der Gedanke.
4. Der Gedanke ist der sinnvolle Satz.
A reasonable translation would be
1. The world is everything that is the case.
2. That which is the case, the fact, is the existence of states of affairs.
3. The logical image of the facts is the thought.
4. The thought is the meaningful sentence.
This suggests two very different layers: the world on the one hand and its logical image, meaningful sentences, on the other.
To decide the question of nominalism vs. realism I think it is important to know a bit more, namely that the German terms have very specific connotations. And a main thing to consider here is that he essentially contrasts states of affairs with thoughts. This strongly suggests that the intention is to contrast material and ideal existence as well since the connotation is that which happens in the objective material world vs. that which happens in the logical idealist world of our thoughts.
Indeed, that is the common reading of the Tractatus: Wittgenstein champions a correspondence theory of truth and at the same time links meaning to truth, ie. only those sentences that are a logical mapping of facts (=true) are meaningful. It also means nothing we say or think is actually a fact in this view, it is always a logical mapping thereof and the two have to be kept apart.
Due to his later discussion of morals and religion, though, some people argue that the end of the book kind of turns itself against how it started off. This is discussed in other question threads, though, and not relevant for the question at hand.