In the video "SMS Pillar # 3 SAFETY ASSURANCE (Safety Management System)", it is explained that safety assurance is the third pillar of Safety Management Systems (SMS) and plays a crucial role in maintaining the highest safety standards in aviation. It encompasses three key components: safety performance tracking and measurement, change management, and constant SMS improvements. Safety performance tracking involves measuring and analyzing safety data, while change management focuses on managing organization-wide changes. Constant SMS improvements are achieved through regular evaluation of facilities, equipment, documentation, and employee performance. Safety assurance is considered one of the most critical pillars for ensuring and improving safety in aviation.