Words do not have any universal meaning. If you don't believe me, take a trip around the world - and I really mean around the world, not just the major cities - and only speak your native tongue. Try to describe to a local the frustration of having to remove all those layers of clothing when entering a warm train after spending a day outside in a Siberian town. Go ahead and explain that you're not sure if the ponderings of Descartes are actually in any way useful to a student of philosophy in Spain. Or just try to ask for directions in Berlin, Pontarlier, or somewhere in Quinqhai. Very, very quickly you'll notice that all those words of yours are, to them, completely meaningless.
Words are mere symbols. A symbol is a thing that by itself is meaningless. If it is to have a meaning, it needs to be given one. That holds true regardless of whether your symbol is a sound, a picture, a gesture or something else.
The meaning of a symbol is the thing to which the symbol refers. The thing being referred to can either exist within or without (outside of) the human mind. The symbol is a sticker that we slap onto the thing. When presented with the symbol, we think of the thing in the world the symbol refers to.
There is nothing that prevents a person from making up his own symbols, or giving existing symbols meanings of one's own. Still, we mostly use symbols that are also used by others. This comes with both upsides and downsides.
The upside is that now the symbols aren't merely something we can use to aid our memory or understand the world, but also something that can be used to communicate with other minds.
This is not a problem if the other person speaks a language you don't know. You know you are being presented with symbols, but you know you don't know the meaning of them. You simply don't understand. You can just let them be. But when another person is actually presenting you with symbols you do know, there is a chance you're not attaching the symbols to the same things. The thought, the idea, is not received as it was sent. The recipient misunderstands.
To not understand and to misunderstand are not the same thing.