What follows is my attempt to "fake it" in explaining Stoic physics. I don't really understand it, so don't take it too seriously; I really am just faking it. It should give you an idea of the flavor, though.
The universe is a sphere. It has a finite radius, but outside the sphere does not exist. The term "void" can be applied to it, but it is more of a verbal placeholder than a reference to something that exists.
The only things that exist are bodies, which are bound three dimensional areas. Every point in the sphere of the universe is included in at least some (and usually many) bodies. (The Stoics were enthusiastically opposed to the atomism.) Note that bodies may (and do) overlap.
There are classes kinds of bodies, including "substance" and "quality". Every quility must overlap a substance, and every part of a substance must overlap at least some qualities. (There are other kinds of bodies, which I don't understand well enough even to fake it.)
Only direct contact (spatial contact or intersection) with a quality can cause events.
Consider an apple. There is a substrate, bound by the surface of the apple. There is also a body of sweetness covering the same area, a body of redness surrounding it, etc.
Consider a knife. There is a substrate, bound by the surface of the knife. There are also qualities, bodies of weightiness, rigidity, and others, covering the same area as the knife. On the surface there would be an body of shininess, and along the edge an area of sharpness.
If I were to cut the apple with the knife, the cut to the apple would be caused by the intesection of the bodies of sharpness and rigidity and the apple.
The human mind is an area covering the human body, the substrate being pneuma, a combination of fire and air (yes, substrates seem to be able to overlap each other, so the human body and human mind can occupy the same space). Human thoughts, decisions, and opinions (also bodies) are qualities intersecting with this pneuma. Pneuma extends not only over our bodies, but over everything in the universe, I think somehow related to all qualities, and a cause to all events. This pneuma is the mind of God. In a sense, when we observe anything that is not our own thought, opinion, or decision, we are watching God's thoughts.
The Stoics also divided everything up into fire, air, earth, and water, following Hericlitus. How there were supposed to relate to substrate/quality/etc., I have no idea.
So, I think you can see why not many modern people take it very seriously (but see here).