Lubricant is an oily or greasy substance that makes machine parts move more easily. If your door squeaks loudly every time you open it, you may want to use some lubricant on its rusty hinges.
When metal surfaces are in contact with each other, especially when they're rough or rusty, there's friction. In order for a machine with moving parts to work quickly and efficiently, it's important to reduce that friction, which is where lubricant comes in. These are usually made mostly of oil, though often they're thicker than a liquid, and they enable two surfaces to slide smoothly against each other, whether that's a key in a lock or engine parts in a car.