The door to the refrigerator has a flexible seal that prevents cold air from escaping when the door is closed. The seal should make smooth continuous contact with the refrigerator case. When the seal does not seal completely, warm air enters the appliance. This results in more frequently compressor operation and possibly the inability of the appliance to maintain proper temperature.
To test the seal, use the dollar bill test. Place a a dollar bill or a piece of paper between the seal and the refrigerator and close the door. Now pull the paper out. You should feel tension as you pull. Retest along the entire door seal.
You should also inspect the door seal for cracking or a lack of flexibility.
If the door seal does not seal properly, the door may need to be realigned or the seal replaced. Also, the door seal should be periodically cleaned to aid in the prevention of odors and to maintain the seal's flexibility.