The ability to monitor a variety of asset characteristics, like reachability and CPU usage, is one of Ops Center's strengths. However, sometimes you'll want to fine-tune this monitoring capability. Maybe you have a group of important operating systems and you want their monitoring rules to be more stringent, for example.
In cases like that, you can use monitoring policies to fine-tune the way that your assets are monitored. A monitoring policy specifies what thresholds are critical, such as reachability being false (duh) or memory usage being higher than 90%. These thresholds determine when incidents are raised on the monitored assets.
By creating a monitoring policy and applying it to an asset or a group of assets, you can effectively tell Ops Center which information about your assets is important to you, so that you see more signal and less noise.
There's more information about asset monitoring in the Monitoring Rules and Policies chapter of the Feature Reference Guide.