Even the concepts that words are pointers to are sometimes so complex that they can defy easy definition. For example, what is a room? In order for there to be a room, there already have to exist in your lexicon a whole lot of other words and concepts. Even then, it's difficult to be precise--when does a box-like space stop being a room and become a box, for example?
Philosophers expend a lot of energy worrying about this stuff. I don't.
I didn't worry about it earlier but somehow now I'm thinking about it too much. I'm not going into philosophy though , I'm trying to think in lines of linguistics and psychology.
EDIT:Taking your example - room and box, what troubles me is that our brains can distinguish between room and box when shown but we cannot clearly express/define what either of them is.
Siimilarly for other things, it is sometimes difficult to define something, but when shown in front of us our brains can clearly tell what it is. Why is this?