a very nice one.. I have one! really like it a lot... and it is great for traveling. I just it was bigger.. but that would kill the carry-on travel use!
Looks to be a padded divider. Use it as you see fit.
For example, if you have a camera/lens in the case, in a vertical position (as the bag is sitting as is), you might want to put the padded divider under it, so that if the case is dropped (or bouncing on stairs etc) the extra padding is there to prevent the gear from slamming into the bottom of the case.