It's a gusset that can be added in to your design to reinforce walls that are prone to warping in a specific direction due to the design of your part.

I was dissapointed to wake up to a 14 hr print where a wall had warped in a strange way (the red part). It actually looks like a y-axis shift, but it isn't. The other 2 walls are fine. Upon closer inspection, I observed that the wall was collapsing towards the back of the machine and 'recovered' once it reached a larger mass section giving the appearance of a shift. I fixed this one with a dremel and some ABS glue.

When it came time to print its mirror sister in pink, I tried adding in some gussets to support the wall that had collapsed. I used 4 of them evenly spaced. It worked quite well. Sorry about the blurry pic. I didn't check it before I cut the gussets off.

This works well when you know exactly where your part is prone to warping.