Maybe it should be called a slideout.

A horizontal dropout is just as its name implies. It allows you to slide the wheel straight out horizontally as opposed to diagonally or vertically to the front as in "normal" dropouts. The advantage here is that if you do have a rear mech failure and need to convert to a singlespeed (or double/triple as the case may be depending on your crankset), being able to adjust the position of the rear axle (and consequently the chosen rear cog) allows you to properly tension the chain.