Thomists have always made a great deal of the word esse. To understand the word well, it’s imprtant to look at the things that St. Thomas distinguishes from esse. We’ll assume non-esse as simply given.
1.) Potency: This is a mode of being that can fluctuate between seeming not to exist at all, and seeming to be the fundamental thing in existence. On the one hand, a single brownie is potentially two, but it seems simply wrong to refer to the last one on the tray as “the last two on the tray”. On the other hand, the universals we predicate of things contain the singulars beneath them only in potency, and yet Hegel argues that one can simply generate all things out of the most confused and imperfect idea of being. We might also point out that our rather silly brownie example is simply a variant of the Zeno’s paradoxes (sc. the inability to get to “the last one” in a continuum) and these paradoxes were so vexing that it took Aristotle to solve them.
2.) Causality. These are distinguished from esse because the most causal of all things, the end, is often acted for when it does not yet exist. The bird making a nest or a hormone released to stimulate growth are both acting or something that isn’t there yet and in fact the nest not being there is a sine qua non of the birds activity. According to St. Thomas, this priority of cause over existence is what caused Pseudo-Dionysius to prefer to call God the good as opposed to the existent, for the good is the most causal of things.
3.) The Actual Existing Substance. This is far and away the easiest thing to confuse with esse. Esse is a principle, sc. it is that principle by which a substance exists in act. Said bluntly, existence does not exist- and if we say it does we fall into the incoherence of an infinite regress.