When a "flush Floor" slide reaches the outer limit of travel, the inner edge drops down over the edge of the floor frame of the trailer so it is "flush". (The originality of naming such features is amazing.)
When it starts back in, there has to be some kind of a ramp to let the leading edge climb back up above the main floor.
The piece you are looking at provides that ramp effect. It is made of ABS (the same as car bumpers) and should be available from a good RV body parts house.
Sorry, I didn't have time for a long answer the first time I posted.
A