Everything rests on something else, but God is that upon which all ultimately rests and in turn rests upon nothing else. It's like objects in relation to gravity on earth. And God is like the earth itself.
A hat rests on a head. A head is part of a human. The human sits on a chair. The chair rests on the floor. The floor is part of the building. The building has foundations. The foundations are secured to the ground. The ground is part of the earth's geology. The planet itself is the ultimate ground of orientation. What does the planet rest upon? The planet does not rest upon something else the way the objects on it rest upon it. It is its own source of orientation.
God lacks contingency in a more fundamental sense, this is just an illustration. But it shows how there can be a series of things that are contingent upon each other, but the ultimate source is itself not contingent upon anything else.
This is among the best arguments for God in my view.