If I remember correctly this means that there hasnít been any current flow through the defrost circuit in 48 hours (not sure on the time it may be 24 hours). What this means is the defrost system isnít working and if you continue to use it without making a repair if will ice up and quit working. This can take anywhere from 2-5 weeks depending on the humidity and how often you open the refrigerator door. It is best if you fix it now before it gets iced up on you.
The problem will be one of three things the defrost heater, defrost thermostat or the defrost timer. Remove the back wall of the freezer to access the defrost thermostat and heater. Check for continuity across the heater and thermostat with an ohmmeter. Note make sure the thermostat is still cold when testing because if it gets above 50 or so it will open anyway so your test will be false. If both the thermostat and the heater check ok replace the timer.