You can apply filters to a selected search category to further refine the search. The filters that are available are appropriate to the type of resource you are searching for.
For example, when searching for servers, you can filter the results by server name, address (IP address of the server), the pool it's in, and the VMs on it. When searching for virtual disks, you can filter the results using criteria such as whether or not they are shared, and the storage repository where they are located.
You can only select filters that are applicable to the resources you are searching for.