An exclusion cage is just a wire cage that you put out in a food plot so that deer can't eat inside the cage. You can see how much the deer are eating from the plot by comparing how tall the plants are inside vs. outside the cage.
While this is true in a general sense, I'm not so sure it tells you 100% how much has been eaten as once plants are bit down to the dirt they tend to get "stunted" and not grow very fast. This could very well make the cage test appear that the plot has produced more for deer than it really has. Still it's worthwhile to do.