I find it useful to make an admittedly idiosyncratic distinction between "being" and "existence". These two words normally have the same meaning—as in, "being" is literally defined in dictionaries to mean "existence"—but giving them different meanings lets me talk in a way that avoids making a certain metaphysical assumption that's baked into the standard definition of existence.

So what is "existence"? Etymologically, it comes from Latin meaning "to stand out". Connotatively, when a thing exists, it does so in a way that allows it to be identified apart from its surrounding context. If no such setting apart is possible, then a thing doesn't exist.

To give a simple example, think about a pancake breakfast. The pancakes exist because you can tell them apart from each other and from the plate, the table, and everything else that makes up the world in which this breakfast is happening.

Now suppose someone said that the pancakes contain caloric fluid. Does caloric fluid exist? It definitely exists as an idea because it's just been expressed as one. But does it exist, really?

We study the pancakes in great detail, using all the latest scientific instruments and methods available to us. We find no evidence of anything worth calling "caloric fluid". At that point we'd say that caloric fluid doesn't exist because, even if caloric fluid is really there, we can't tell it apart from the rest of reality, much in the same way an invisible dragon doesn't exist except in the mind of the person who believes it's there.

Unfortunately, the normal way of understanding "existence" smuggles in a metaphysical assumption, namely that things exist independent of mind.

I'm not going to argue that things don't exist independent of our minds. However, we should be clear that this is a metaphysical assumption being made about the world. It's metaphysical in that no amount of physical evidence can prove that things are really real. The physical world as we experience it is equally well explained by it being real, a simulation, thoughts in the mind of a Boltzmann brain, or a highly self-consistent hallucination. So when we say that something "exists" and mean that it "really, objectively exists independent of anyone's beliefs about it", that's assuming a particular metaphysical view.

The reason I'm not going to argue against this metaphysical assumption is because it's a really useful one for modeling the physical world and because there's no point in arguing metaphysics since it's literally beyond our ability to know. But I do think we can speak more carefully without making this assumption. That is, we can keep our conversation on the purely physical level if we simply change slightly what "existence" means and give "being" a meaning other than "existence".

First, though, a few words on "being". You'd think this is the older word, given that it's constructed from "be", but it's not. First attested circa 1300 CE, it seems likely a word created to mean the same thing as "existence" in English. Not that things didn't exist before in English, but there's a good chance there was simply no word to talk about being/existence because everyone who wanted to talk about this concept did so in Latin. English-only speakers, who were mostly poor agriculturalists at the time, just said how things are, with no bracketing to talk about things are-ing.

This makes "being" ripe for redefinition, especially because, unlike "existence", the etymology has nothing to do with standing out. It has to do with what is, which, when we drop the metaphysical assumptions, becomes something different from existence.

As I prefer to use the words, "existence" always means "ontological existence". This fits best with its standard definition. In fact, it's exactly the same, except it's cutting out the part where we jump to "and therefore that means a thing really exists in external reality". In other words, it's acknowledging only the phenomenon and saying nothing about the supposed noumenon being referenced.

For the most part this doesn't change everyday speech. But it does give "being" space to mean something else. Specifically, we can use it to talk about the way things are when they have not yet even become things because they have not been reified into existence by a mind. This is the being in which the whole world simply is without any conception of it made by us or anyone else.

Being able to talk about the distinction between "being" and "existence" is important because we constantly experience the world without reifying it. We do this every moment. It's only in a small number of moments that our minds turn their attention to things and bring them into ontological existence. The rest of the time we are simply interacting with the world without conceptualizing it.

When we conflate "being" and "existence" we lose a way to talk about the world without smuggling in metaphysics. This is unnecessary, and having these two words mean different things lets us speak more clearly and carefully and helps avoid the confusion created by baking a metaphysical claim into the word "existence".