It means everything to everyone.

However essentially a world model is something that has the understanding of 3d world and can generate novel view point given either text or image input.

The use I have seen is for robotics. You feed in the current view and describe the action you want it to do, and then it plans the arm movements. (really useful for softbody manipulation.

There are other meanings. but essentially a world model is able to reason in 3d, rather than text.