Filtering a report allows you to just show specific items in a data set, like data for just one project or just one discipline. Selecting multiple filters will aggregate data for all of the filters you select. You will only see filters that are actually available in the data you have grouped.
Clicking on the name of the field option is a quick way to filter down to only that value. For example, if you click on "Fulcrum" in the Client field, it will deselect everything else and only select that one client. Clicking the checkboxes to the left allows you to select multiple Clients.
On the Project Page, you can use the quick links to access project reports, and automatically filter the report to its Time Frame, Project Name, and Client.