The oil in the engine serves various functions, it lubricates the moving parts, cools & transfers heat and cleans the engine. There are basically three types of oil available: Monograde, Multigrade and Synthetic, the former being the oldest. The first two are Mineral based oils with varying characteristics.
A measurement of the oils ability to flow is called viscosity, and this is shown on the can as a figure. This is measured at normal working temperatures, so a monograde will be a lot thicker when it is cold. To get around this thinner oils could be used in very cold climates or during winter periods. A modern Multigrade oil has additives that stop the oil thinning as quickly as a straight oil as it gets hot, so it gives better protection at all temperatures.
Technically, a modern Multigrade oil at 0 degrees Celsius the oil viscosity rating is compared to an equivalent straight oil’s viscosity and similarly, at 100 degrees Celsius, it is again compared to a straight oil’s viscosity at 100 C.