It didn't seem like it did anything, so I reluctantly cut it out. I installed Dynamat, and the door panel wouldn't go back in with this thing bulging out of it. It's hollow. One in each door. I Googled the numbers on it and didn't turn up anything. Thanks!
http://tinyurl.com/8wmdonv

 

Its hard to tell but it could be protection for the guide rods from opening locked doors.
Try going to a Honda parts dealer like Majestic Honda and then enter the part number to see what it is if you can't see it on their Fit parts list on the doors.