Oil Grades
This means the viscosity is at 10W when the engine is cold and 30 when the engine is hot.
Low viscosities are good for cold temperatures (hence the "W" association!) because the oil is thinner. Thinner oil flows more easily and moves quickly. When you start a cold engine up again, oil needs to travel to the top of the engine, where it trickles back down. Since oil is vital to lubricating your engine, it needs to move quickly and low viscosity helps it do just that.
But when your engine heats up, higher viscosities mean the oil won't thin out too much and will keep parts separated by a film of oil.
So your oil is always maintaining a delicate balance: it needs to flow well when the engine is cold but also retain enough body at higher temperatures to keep metal parts lubricated and separated. To find out what oil grade is right for your car, check your owner's manual.