What exactly does this mean? I've looked up various sites, but they've failed to provide me with an explanation of how science has proved this, or what exactly it means... other than the fact that it is limited and does not expand. [I also read somewhere that it was flat and shaped like a triangle?]

Also why use the term flat? I'd prefer "constrained" because flat makes me think of something 2-dimensional, when that is not the case of the universe. Is it because, if you go in one direction, you'll never return to your starting point like on a globe? Can someone fill me in please.