Your home loses and gains heat three ways- convection, conduction and radiation.
Convection is the transfer of heat by moving air. Without an air impermeable insulation, air can leak through exterior walls and ceilings, wasting energy and precious warmth in the colder months.
Conduction is the transfer of heat through a solid material, such as a ceiling or a wall. To combat conduction and keep your house cool in the summer, go with a insulation that maintains its R-value over a wide range of densities.
Radiation is the transfer of heat in the form of electromagnetic waves. A home equipped with proper insulation will help reduce the negative effects of radiated heat transfer.