You might as well ask what it means to speak of any other basic reality. What does it mean to speak of time, space, number, particularity, universality, good, evil, or other minds? We use such concepts, and accept and comment on the things they denote, but explanations of exactly what they are never completely satisfy.
Still, one might ask why we need to speak of God. Some people say they do without Him, just as some people say they do without good, evil, universals, time, space, matter, and other minds. Some say there is nothing but the One, others nothing but Spirit, others nothing but Atoms and the Void. What do people miss who cut out whole categories of being?
In the case of those who cut out God, I think they’re missing out on the possibility of a world that’s coherent and makes sense overall—in other words, a world we can discuss without talking nonsense. If we are able to talk about the world, and understand it to some degree, then our categories of thought must be somehow in line with the categories of being. The world, in fact, must be a rational structure. How can that be, though, if the structure of things is not thinkable? But if the structure of things is thinkable, doesn’t that mean it has some sort of essential connection to thought?
The traditional proofs for the existence of God add to that perspective. Many of them are based on the thought that an explanation with infinitely many terms is no explanation at all. We understand things by putting them in a setting, but if we need an infinite number of settings then we won’t understand them at all. If that’s right, then the world is rationally comprehensible only if there is some ultimate explanation that explains itself and all other things. Reflecting on what such an ultimate explanation would be like gives you many of the traditional attributes of God—one, all-powerful, self-caused, pure act and so on.
In addition, denying God makes it harder to have an ethical theory that makes sense. “Good” means whatever it is that makes something a rational goal of action. If there’s no good then no action is more rational than any other. That’s not what we believe, though. Further, reason—which necessarily includes an understanding of the good—isn’t something we invent. If it were it couldn’t serve as a standard for thought and action. So the good can’t be something we invent either. It’s somehow implicit in the nature of things. But how can that be unless subjectivity and purpose are implicit in the nature of things? A tree might fall in the woods without anyone to hear it, but it’s harder to imagine how goods could exist without a subjectivity and will for which they are goods.
All of which will seem like nonsense to those who do not already partly accept such views. That’s the nature of basic points: you don’t derive them from other considerations. They’re more a matter of Newman’s illative sense or Pascal’s intuitive mind, of what comes into focus on consideration of experience, than the latter’s mathematical mind. Still, you can’t avoid them in dealing with life. So the standard for accepting them, it seems, is what you and others generally find necessary in understanding and otherwise dealing with the world. In questions like the existence of God, consensus gentium really does seem to be an argument.