Cloudera Manager's activity monitoring capability monitors the MapReduce, Pig, Hive, Oozie, and streaming jobs, Impala queries, and YARN applications running or that have run on your cluster. The Activity Monitor provides many statistics about the performance of and resources used by those jobs, queries, and applications. You can see which users are running jobs, queries, and applications both at the current time and through dashboards that show historical activity. When the individual jobs are part of larger workflows (via Oozie, Hive, or Pig), these jobs are aggregated into activities that can be monitored as a whole, as well as by the component MapReduce jobs.
If you are running multiple clusters, there will be a separate link under the Activities section of the Clusters tab for each cluster's MapReduce activities, Impala queries, and YARN applications.