10W-30, Represents two SAE numbers, 10W and 30.
10W represents the first SAE number and is associated with the SAE number for cold temperatures. An SAE number is a measurement of the viscosity or thickness of oil. Why is this number important? This number is important because in cold temperatures you need the viscosity to be thinner as oil is usually thicker when cold and therefore will not be able to properly lubricate the engine and its components. The lower the first SAE number, the thinner or more viscous the oil will be at lower temperatures or during startups.
30 represents the second SAE number and is associated with the engine running temperature or at 100-degree celsius. Why is this number important? This number is important because when the engine is running, the oil is hot and therefore already thin or viscous and therefore if this number were to be any lower, say like 20, then the oil will be too thin and will not be able to effectively lubricate the engine and its components.