Basically, it is a computer controlled machine that builds parts out of material by adding several layers over and over until a 3 dimensional “thing” is produced. It’s pretty cool stuff and it is becoming more and more popular among scientists, engineers and professionals. It used to be very expensive and not many people knew how to use it. With the popularity and ease of use of many 3D solid software packages and off-the-shelf electronics, it is becoming much friendlier to the hobbyist. The MakerBot retails for under $1000.
In fact, some futurists believe that one day, rather than going to the store to buy certain things, you can go to your machine and “print out” what you need. That technology is far off in the future, but here is an example of someone using their MakerBot to create Lego pieces.
Here's a video of the machine in action.