Friction is the resistance that one surface or object encounters when moving against another, which restricts them from sliding or rolling over one another. So two unlubricated surfaces, for example the teeth of two interlocking gears, are slowed down by friction.
While they appear to have smooth surfaces from a distance, they're in fact rough at an atomic level – meaning one surface drags against the other, which wastes energy and wears out the materials.
Lubrication controls friction and wear by introducing a friction-reducing film between moving surfaces, so a lubricant such as grease helps reduce friction in two ways. Firstly, it smooths and cushions the bumps between the two surfaces and, because it's a liquid, can change shape and flow.