An insulator is a substance which does not readily allow the passage of heat or sound.
A common example is the insulator flask found generally at each home which keeps drinks hot or cold by providing an insulating layer between the drink and the outside air. Air is a poor conductor of heat, so the tiny pockets of air trapped in insulation minimise the amount of heat which can pass between the inside and outside of your house.
How the Insulator works
Most of the insulation works by trapping material which are poor conductor of heat in a closed space i.e. Air, Ice. When we trap these material in close space i.e. wall of the flask in above example it reduces the ability for heat to be transferred by conduction.