Depending on the service, the Summary tab includes a Metrics dashboard that is by default populated with important service metrics to monitor:

If you have the Ambari Metrics service installed and are using Apache HDFS, Apache Hive, Apache HBase, or Apache YARN, you can customize the Metrics dashboard. You can add and remove widgets from the Metrics dashboard, and you can create new widgets and delete widgets. Widgets can be private to you and your dashboard, or they can be shared in a Widget Browser library.

You must have the Ambari Metrics service installed to be able to view, create, and customize the Metrics dashboard.