The ambient temperature is the temperature of the air surrounding your food. If the barbecue lid is open, ambient temperature measurements won't be reliable. However, if you have the lid closed, you can monitor the temperature to ensure your heat source is still active and where you need it to be while barbecuing or smoking. It's also a useful measurement for where you put food -- as heat will likely be higher closer to the source, and it where it is cooler, the internal temperature of items will take longer to reach a safe level to eat.