"Ex nihilo nihil fit" a statement that was not first stated by Thomas Aquinas but by Lucretius. Anyway, that's not the point, the meaning of this statement is that everything can never come from nothing there would always be something that caused it. Considering the 5 Proofs of Aquinas you would see there, from the proof of Contingency, a proof that there is a necessary being or a cause in which everything begins with. The statement also means that everything here on Earth exists because of a reason or an end. Everything that exists, every event that happens, every situation that we are in have a cause and that cause is nothing.

That's all thank you hope this helps!