Scientists, or at least some of them, believe that the universe may consist of as many as eleven dimensions - the three with which we are familiar, a fourth dimension (time), and seven additional dimensions which are "folded up" so that we can't perceive them.
To grasp what's meant by "folded up" imagine a piece of paper representing a two-dimensional plane. Now imagine that the sheet is rolled up very tightly so that it looks like a tube. Imagine that it's rolled up even more tightly so that it looks like a wire. If we could do that to the paper we would have converted it from a two-dimensional plane to a one-dimensional line by folding it. The second dimension is still there but it's "folded up" so we don't perceive it.
Not only can't we perceive dimensions beyond the first four we can't very well conceive them either, but the first 12 minutes of this video by Rob Bryanton explain how scientists conceptualize them. The video is helpful, but Bryanton moves quickly and doesn't allow much time to think about what you've heard so be ready to back it up from time to time and review it.
Exit question: What's the point of these extra dimensions if they're imperceptible? What function do they serve?
Thanks to Michael Weiss for the link to the video.