The Other Side is the mythical realm of souls, spirits, ghosts and demons. It is rumored the Nether Dragon comes from there, and in the past the Smoke Dragon. It is rumored to be inhabited by some non-dragons too, like Firemanders and the mythical Lava Golem. There is a good side and an evil side: most dead people go to the good side, the evil people go to the evil side (also known as the netherworld), which is also inhabited by devils and demons. The Ancients always tried to communicate with The Other Side.
Other Side

A painting of Pablue Pickaxe depicting a castle in the evil side of The Other Side