Air Traffic Control (ATC) is the ground service that manages and separates manned air traffic: the airport tower, approach control and area control. These are the controllers who give pilots radio instructions so that aircraft stay clear of each other and do not collide.
An open-category drone pilot does not talk to ATC on the radio. Yet it is ATC's very existence that defines which airspace is controlled — for example a control zone (CTR), where drone flights need coordination through the geo-zone process.