From SKYbrary Wiki
The role of the ATCO is to ensure the safe and expeditious flow of air traffic through the airspace for which he/she is responsible. The controller must remain alert and effective throughout his/her period of duty, whether by day or by night. At all times, he/she must be able to cope with additional unforeseen situations, such as an aircraft emergency, or extreme weather conditions.
Put simply, situation awareness (SA) means knowing what is going on around you. More specifically, in the context of complex operational environments SA is concerned with the person's knowledge of particular task-related events and phenomena. For example, for a fighter pilot SA means knowing about the threats and intentions of enemy forces as well as the status of his/her own aircraft. For an air traffic controller, SA means (at least partly) knowing about current aircraft positions and flight plans and predicting future states so as to detect possible conflicts. Therefore, in operational terms, SA means having an understanding of the current state and dynamics of a system and being able to anticipate future change and developments.