Motion Capture, or Mo-cap is a technique of recording realistic movements. It is used for movies and video games.
Here's a basic, stripped-down, vague explanation: Instead of having the artists in a video game animate the 3D models from scratch, they hire someone to wear a skin-tight black suit covered in ping-pong balls at specific joints and tell them to move around. These movements are recorded onto the magical computer, and then they do something that makes the models move just like the recorded information.
Motion capture is an expensive process, but it gives the most realistic animation possible.