The basics of occlusion culling

Occlusion in a Unity scene is data that divides the world into chunks containing smaller and smaller chunks of space. In each of these sections, the data for what is and isn't visible from each section is recorded. Unity takes this data and uses it to hide things from being rendered.
It's important to remember that occlusion will only hide things visually. Most things will keep on running when hidden, so occlusion culling is strictly for reducing draw calls. Also, Unity won't hide objects it thinks are visible, no matter how far away they are. Occlusion culling doesn't care about distance.
Wait, occlusion culling is only visual?
That's right - occlusion culling only stops things from rendering. It doesn't stop them from doing any processing, so if you have a big world split into sections, you can gain performance by disabling parts you know aren't currently active.