The Anchor Sentinel is a machine from the Railcraft mod. It is used with the World Anchor to extend its chunk loading capabilities. It can only extend it by 9 chunks, and in a straight line. Only one Sentinel can be linked to any given anchor. It will not add more chunks to be loaded, it will simply change the shape of the loaded chunks from 3x3 to 1x9.
Use a Crowbar to initiate anchor pairing. Next, right-click a World Anchor that is within 9 direct chunks of the Sentinel with the Crowbar. "World Anchor was paired successfully" should appear in the game chat.