A performance testing tool is software used to evaluate the speed, scalability, and reliability of a system under various load conditions. These tools simulate user traffic to identify potential bottlenecks or weaknesses in application performance. Performance testing encompasses different types, including load testing, stress testing, and endurance testing, to measure how systems handle varying levels of demand. The insights gained from performance testing tools help teams optimize applications for better user experiences, ensure scalability during peak times, and maintain system reliability and availability.