R-Value is a measurement of the thermal resistance of building materials. It provides a way to standardise the amount of thermal resistance required to insulate a structure. R-Values are calculated by the amount of time it takes for heat to travel through a material at a certain thickness.
Ceiling insulation works the same way as all types of insulation. Insulation works by trapping air particles inside of lightweight, fibrous materials. By placing this material between two areas this slows down the speed in which heat can transfer between them.