Final answer:
A PAC alert on a radar screen warns pilots of nearby aircraft and potential collision risks, providing advice on how to safely adjust their flight path if necessary.
Step-by-step explanation:
A PAC alert on the radar screen warns the pilot of proximity to other aircraft or potential collision risks. It stands for Proximity Advisory/Collision Alert and is part of an aircraft's Traffic Collision Avoidance System (TCAS). When this alert is displayed, it means that the system has detected another aircraft in close proximity, and depending on the level of threat, it may also provide resolution advisories which advise pilots on how to adjust their flight path to maintain a safe separation.