Lacus Felicitatis is a small patch of the lunar surface that has been inundated by flows of lava, leaving a level patch with a lower albedo than the surrounding ground. About 70–80 km to the northeast of this area are the Montes Haemus, along the southwestern edge of the Mare Serenitatis. In outline it has a bent shape, with a wing to the northwest and another to the east. (wikipedia entry)
A lacus is a "lake" or small plain. On the Moon they are named after latin terms describing weather and other abstract concepts.