While others explained what a prim is, i add some addition to it.
Changing an object Physics Shape Type to reduce land impact might also change the object behavior.
Example i have an mesh wall with center hole for a door. When the object is set to prim, it allows you to pass but when set to convex, it behaves likes its a full wall, even if it has a hole in it for a door, you cant pass.
Scripts, textures, size ect can affect the land impact. Not only that, but also linking or unlinking a mesh can cause instantly the land impact to rise up to ~300 LI or more. And yes, that will