For food that is not fully cooked, you may not remove and replace the lid (such as when checking to see if the food is cooked).
Reason Replacing the lid helps cook the food (the food's being fully cooked before Shabbat avoids this problem).
When using a crockpot or slow cooker with a glass lid, however, the food does not need to be fully cooked.
Reason You can see how well cooked the food is through the lid.