The inner cover separates the outer cover from the rest of the hive and makes it easier to get the outer cover off if it gets stuck. Sometimes inner covers have a cutout on one side of them that acts as a second entrance. These are only able to be used with a telescoping top, not a migratory top (see Outer Cover for more details).
This inner cover lacks a cutout. See Hive Parts for more details on this entire subject or click a link here to find out more about a specific part, Outer Cover (specifically a telescoping top in this picture), Hive topper/Vent box, Boxes (Brood chamber and honey super together), Bottom Board (specifically a screen bottom board in this picture).