When two solid surfaces slide over each other, friction acts between them. Friction does work against the motion and transfers energy to the thermal energy store of the moving parts and the surroundings. This energy is wasted because it is not useful. Friction also rubs tiny pieces off the surfaces, which causes wear, so the parts wear out sooner.
A lubricant, such as oil or grease, is a liquid or slippery substance placed between the moving surfaces. The surfaces then slide over a thin layer of lubricant instead of rubbing directly on each other. The friction becomes smaller, so less energy is wasted as heat and there is less wear. Lubricants are used on bicycle chains, door hinges, axles, gears and inside car engines.
The wasted energy can be compared as a rate. If a bearing without oil wastes 60 J as thermal energy in 20 s, the rate is 60 / 20 = 3 J every second. Adding oil lowers the rate, which means less energy is wasted each second. Lubrication reduces the unwanted energy transfer but it cannot remove friction completely.