A lubricant (or lube) is a material which can be put in a cube to make it easier to turn or smoother. In general, to lube a cube, you should apply the lubricant either by removing one piece and spraying it in, or by disassembling the cube completely and coating the pieces in lubricant.
After lube is applied, it is usually best to not time a solve immediately, but rather give the lube a few minutes to work in. It is also recommended that you do not apply lube to very stiff cubes, as this can sometimes make them even harder to turn; instead, try putting vaseline in the cube, then after a week or two, clean it out and then lubricate it.
This Gan Magic Lube 10ML (2 Pack) is the perfect viscosity to balance speed and longevity.