The Traffic Collision Avoidance System (TCAS) is an aircraft collision avoidance system designed to prevent mid-air collisions and is mandated for aircraft over 5,700 kg. It operates by building a 3D airspace map using interrogation and response from transponders, issuing threat advisories (TA) and resolution advisories (RA) when necessary. TCAS can be classified into two types based on passenger capacity, and pilots are required to follow RA maneuvers, which may override ATC instructions.