Describe an app idea in a sentence. We read it, search a database of App Store apps for the ones that do something close to it, and add up their estimated monthly revenue. What comes back is the size of the space your idea sits in, plus the list of apps already competing there.
It is a measure of how much money the apps nearest your idea already take. It is not a prediction of your revenue, and it is not Apple's data. Apple publishes no per-app revenue, so figures like these are modelled estimates and coverage is partial: many apps carry no estimate at all, which pulls a category total down without meaning the category is small.
Read it alongside movement. A category worth a lot but flat is a harder place to enter than a smaller one that is climbing — see which apps are rising and the current App Store charts by country and category.