In your PCB layout you define a side of your PCB as TOP. Hence layers present on that side of you PCB will be called TOP xxx. Examples are TOP circuit layer, TOP solder mask, etc… Read our 10 rules for better data.
The TOP side of the PCB is usually the side where the components are placed and therefore sometimes also called COMPONENT side. However more often, PCB are populated on both sides and thus is the naming TOP more clear as definition