Performance Monitoring is a set of processes and tools to be able to determine how well fast applications are running in the cloud. Due to the nature of cloud applications being highly distributed, special software is required to be able to trace where traffic is going between networks and applications to truly understand how well an application is performing. There is an entire industry dedicated to measuring how well cloud applications, infrastructure, and networks perform, called Application Performance Monitoring (APM). APM tools will tell you the latency a user may expect on your website, or where a bottleneck in your network is based on telemetry it is measuring. APM tools are installed by developers and infrastructure engineers, and operations teams monitor these systems that are live to make sure all systems are healthy.
in other words
When your website brings home its report card.