The Coverage Render Element holds anti-aliasing data based on the Render ID Render Element. When the Coverage Render Element is multiplied by the Render ID Render Element in compositing, it adds anti-aliasing to the Render ID channel, providing a quick object mask.
To use render elements, select from the Available render elements column and click the Add button to add the passes you wish to generate at render time. When you render the scene, the added render elements listed in the column on the right, will also render, which can be viewed from the VFB window's channel drop-down menu.
For more information on the parameters on this dialog, see the Render Elements tab page.