A service-level agreement (SLA) is significant in IT services as it clearly defines the expectations and responsibilities between a service provider and the client. It outlines key metrics such as response times, uptime guarantees, and support protocols, which are crucial for maintaining service quality. SLAs provide a framework for accountability, allowing clients to measure the service provider's performance against agreed-upon standards. By having an SLA in place, both parties can avoid misunderstandings and build trust, ultimately leading to improved service delivery and client satisfaction.