Multigrade oil has almost the same viscosity at low and high temperatures as do single-grade oils designed for those temperatures.
Single-grade oil like 10W or SAE 30 oil has a high viscosity when cold and a lower viscosity when hot.
The first number 10W is the viscosity of the oil at cold temperature, and the second number 30 is the viscosity at 100 °C.
A multigrade oil like10W30 has the same viscosity at cold temperatures as SAE 10W oil.
At low temperatures, 10W30 has the viscosity of 10W. SAE 30 would be too viscous to use.
As the engine heats up, the viscosity changes to that of SAE 30. SAE 10W would not be viscous enough at this temperature.
This means that one type of oil works in all temperatures.
The diagram below shows the viscosity behaviour of 10W30 oil.
The video below explains more about the viscosity of motor oils.