While a gimbal can be any support that can pivot around an axis, most gimbal systems look like a series of concentric rings. The outermost ring mounts to a larger surface, like a boat's instrument panel. The next largest ring connects to the outermost ring at two points that are perpendicular to the outer ring's surface mount. Then, the third largest ring mounts to the second largest one at two points perpendicular to the connection between the first and second ring, and so on. Sound confusing? Take a look at the following illustration.
Each ring can pivot around one axis. How is this useful? On its own, it's just interesting to look at. But by mounting an object to the center of the system, you can make sure the object can face any particular direction at any time.
Well, almost any direction at any time. One problem with gimbal systems is gimbal lock. Gimbal lock occurs when two axes in a three-gimbal system align. When that happens, the object's movement is limited. An entire range of motion becomes impossible. This is what you see on the right in the above illustration.
Gimbal lock is a serious problem. There are two ways to avoid gimbal lock. One is to adjust the gimbals, either by maneuvering the surface so that the gimbals swing another way or by physically resetting the gimbals themselves. If gimbal lock does occur, the gimbals must be reset to work again. Another solution is to add more gimbals to the system. Adding a fourth gimbal helps eliminate gimbal lock, but it also makes the system bulkier and more complicated. Since most gimbals are part of electronic systems, adding more complexity is not always the best choice.
Gimbals allow designers to create devices that are more flexible than a fixed, stationary device. It's also possible to orient a device so that it's facing a specific direction independently of how its surrounding environment moves or changes. Such an application has dozens of uses, ranging from a cup holder that adjusts so that you don't have to worry about spilling your coffee to an array of satellite antennae that can turn to face incoming signals.
So what does this have to do with NASA? Find out in the next section.