They are inserts, which is kind of like an effects loop on a guitar amp in that it is meant to interrupt the signal path, send it out to something and then return back to continue the path. I can't find a manual for that unit (didn't try too hard
), but I'd guess that it comes after the gain knob and before the ADC.
Usually, you'd plug a TRS>Dual TS "insert" cable, which is really the same thing as a stereo breakout cable where the T of the TRS goes to the T of one TS, with the R going to the T of the other (and the S shared by all three). The one connected to the R should be the send and the other is the return.
I think most commonly you'd use this for something like an outboard compressor that you wanted to apply on the way in. You'd connect your mic or other source to the input as normal, then it would go out the R to the compressor's input, from it's output to the T, and then on to ADC. You can, of course, use any processor (or chain thereof) there that you want.
It really has nothing to do with Reaper itself. It is used to apply external processing before the signal gets to Reaper.