It's basically your character's mind-- anything/everything you want to include in it (memories, motivations, dreams, etc.)-- that people can walk around in. Maybe examples of previous characters' dreamscapes would help?
People literally get into them by falling asleep. When you sleep, you enter the world of dreams, and since dreams are often associated with the subconscious/unconscious mind (along with whatever has influenced us consciously), you've basically entered the world of minds. Since dreams are intangible, there aren't really any boundaries between the minds in that world, and so you could be in Toph's mind, come upon a memory of Mako, and boom, suddenly end up in Mako's mind.
Just think of your own dreams, and how you can suddenly jump from one place to the next without actually having dreamed about how you got there.
Re: FAQ
People literally get into them by falling asleep. When you sleep, you enter the world of dreams, and since dreams are often associated with the subconscious/unconscious mind (along with whatever has influenced us consciously), you've basically entered the world of minds. Since dreams are intangible, there aren't really any boundaries between the minds in that world, and so you could be in Toph's mind, come upon a memory of Mako, and boom, suddenly end up in Mako's mind.
Just think of your own dreams, and how you can suddenly jump from one place to the next without actually having dreamed about how you got there.