Fundamentally, the Five Ways rest upon the distinction between what something is and that it is. This distinction is both within ourselves and within all the objects of our experience. I can know what the do-do bird is while also knowing that it does not exist anymore. The do-do bird still has a what but it does not possess a that. All of creation is a composition of what and that. The what is what gives definition and limit to things. It is what separates an atom from an animal from a planet from a universe. The that is actual existence or being.
Notice that if something is a composition of a what and a that, its existence must be explained by something outside itself. All of our physical reality, though, is made up of this type of composition. Things are constantly coming in and going out of existence; we all will go out of our current mode of existence when we die. Existence is a gift given to all things. That is given to what in order for something to actually exist.
Ultimately, though, we must reach a Giver of Existence. We must reach something that does not share in existence or even merely have it. In order to properly explain the existence of all things we must reach a Being whose what and that are identical. What it is to be this being is simply that He Is. His definition is that He Exists.
When Moses asked God for His Name He replied:" I AM."