Re: What is this?
Its probably there to stop the roller brackets from tilting too far and allowing the bow to contact the trailer frame. Sort of like a kick stand effect. My Shoreline has one on the frame that's adjustable, it attaches on each side between the roller bracket pivot bolts.
When the trailer is on the ramp, and the boat is just making contact, it keep the rollers from just pushing downward letting the boat make contact with the face of he roller brackets or frame.
The roller brackets on mine rest on the stand, thus keeping them at the right angle to start the boat up onto the trailer.