When you first access the Process app, the Filter sidebar is hidden. To apply filters to the Process List, you need to show the Filter sidebar. See Applying Filters from the Filter Sidebar.

However if you do not need to access the features in the sidebar, you can collapse it to increase the screen space for the Process List.

When you dock the Process app while the Filter sidebar is shown, the sidebar is automatically collapsed. Undocking the Process app also restores the Filter sidebar. When you dock the Process app while the Filter sidebar is collapsed, undocking does not restore the sidebar. For more information on docking apps, see Docking Apps.