Consider the question "What is hot?"
At a basic level, we can answer this question by listing things we experience that can become hot. Water, wood, metal, etc. are all examples of this. More generally, we can answer it by saying that something is hot if it has heat. Notice the distinction that now arises. A metal, for instance, can become hot but it can only do so if it receives heat. But heat, in a sense, is also hot. Heat, though, does not become hot upon the reception of some other thing. Heat is hot of itself, and it is through participation in heat that other things become actually hot. Put simply, heat does not need to have heat because heat is heat. It is not hot accidentally but essentially. Therefore, it is the cause of heat in all other hot things.
Being is analogous to this. All things which are have being but only in an accidental way. Returning to heat for a moment, we know that wood or water cannot be the per se cause of heat even if they are hot because neither of these substances are necessarily hot. They are indifferent to heat, at one time having it and at another time lacking it. In a similar way, the objects of our experience at one time exist, i.e. have being, and at another time do not (death, decay, change, etc.). These are things that have being, but having being is not enough to be the per se cause of being just as being hot is not enough to be the per se cause of heat in things. What actual existence requires, then, is a cause which does not merely have (participated) being but, in fact, is Being Itself.
In God there is not distinction between essence and existence.