Non-functional requirements are criteria that define how a system should perform rather than what it should do. They focus on aspects such as performance, security, usability, and reliability. These requirements help ensure that the system meets user expectations and operates effectively in various conditions. Unlike functional requirements, which specify specific behaviors or functions, non-functional requirements address the quality attributes of a system. For example, a web application may need to load within three seconds, be accessible to users with disabilities, and maintain a certain level of security against threats.