A reredorter is a fancy word for a special type of bathroom that was used a long, long time ago. It was a private room that people could go to do their business and it was usually attached to a big building like a castle or a monastery.
Now, the reredorter was a little different from the bathrooms we have today. Instead of flushing everything away with water and pipes, people in the past used something called a chamber pot. A chamber pot was like a little bowl that you could pee or poop into and then you would have to empty it out later.
So in the reredorter, there would be a special place to store all of these chamber pots until they were ready to be emptied. This storage area was usually located underneath the bathroom and was called a "rere-dorter," which means "behind the latrine."
It might not sound like the best way to go to the bathroom, but it was a lot better than using a hole in the ground like people did in even earlier times. Plus, having a private bathroom was a luxury that only the very wealthy or powerful could afford, so having a reredorter was a pretty big deal!