A gimbal is a device that is used to enable the capture smooth motion video. You may have experienced the difficulty in getting your video to look professional whilst walking. Without some form of stabilisation the film looks very jerky and is often quite horrible to watch.
There are many tools you can use to capture smooth motion depending on your budget and location. You can use dollys, which is when the camera tripod sits on top or tracks and glides along the path of the track. There are also stabilisation systems that use weights and sometimes a harness to counter balance the effects of movement ( just like a suspension system). The modern gimbal is an electronic stabilisation system that uses motors to counter act your normal movement. In the hands of an experienced professional this can create some spectacular footage.
This footage was taken with a gimbal whilst I ran down a grass verge only looking at the camera screen. I was particularly pleased with two things after I'd shot the film. 1. how smooth the footage turned out using the gimbal and 2. that I'd managed to stay upright and not ended up face first in the grass!