Random question about uploading a world...how do I define what's a solid object that an avatar can walk on and what is not?

for example if I add a 3D plane to the game world, it's already something an avatar can walk on...but say I upload a large rock that I just got as an FBX from outside unity and import it in...is the entire surface area of the rock just automatically a "solid" surface that avatars can walk on? or do I have to define that "solid" surface somehow?

I've been in worlds before that have walls and objects that I literally can just walk right through, so obviously there's a way to make an object not actually be solid and have 0 collision