The Other Side is a fan-dubbed term for an event that occurs during Batman: Arkham City. The exact nature of the Other Side remains uncertain, however, it is considered to be the Arkhamverse's version of the afterlife. It is where characters go when they die. It could also be where the deities of the Arkhamverse (assuming there are any) reside.
Note: It is debatable as to whether or not the Other Side is real or if it was simply a hallucination on Bruce's part. Though the latter option is the most likely. Regardless of whichever one is the case, the Other Side should at least warrant a mention as it plays an integral moment in Bruce's life when he is closer to death then ever before.
Arkham City Incident
The Other Side's first and only real appearance was for a brief moment in Batman: Arkham City, when Batman was on his way to Wonder City to find the League of Assassins and get Ra's blood for the cure, he stumbled and fell just a little on the way. Briefly succumbing to his illness, Bruce lay there before he saw an impossible sight.
Seemingly only a few feet away, in bright white light, stood his parents Thomas and Martha Wayne in front of some pearly gates. They claim that they have been waiting for this moment for a very long time and try to persuade Bruce to cross over into the afterlife with them.
Although extremely tempted, Bruce is unable to accept their request believing that he was still needed. After this, Thomas and Martha vanish along with the gates and light and Bruce is able to recover his health long enough to make the rest of journey to Wonder City.
After this, the Other Side is never seen again. Although there were various possible references to the place throughout the series.
Assuming it acts as the afterlife for the Arkhamverse, it can naturally be assumed that any mortal will eventually become a resident. It is also likely the home of any and all deities that may exist within the Arkhamverse,