Overview
Activity Monitor displays information about current SQL Server processes and how these processes affect the current instance of SQL Server. Activity Monitor is mainly for monitoring SQL Server with respect to databases and their queries and other measures to help understand what is impacting SQL Server at the current time. This can help you troubleshoot issues as well as see how SQL Server is using server resources.
Explanation
To open Activity Monitor, right-click on the SQL Server instance name and click Activity Monitor. As you can see below, there are several types of information you can review, such as processes, resource waits, expensive queries, etc.

Additional Information