Dreamwalking is Isabel’s special power on the show. We don’t see her dreamwalking that frequently, and all the rules aren’t clear. She needs to touch a photograph of the person whose dream she wants to walk into, and then falls into some kind of sleep or trance state herself. Then she can see the dreamer and the things and people he’s dreaming about, (even dream versions of herself,) and can speak to the dreamer if she wishes to.
During the show, some of the other aliens encourage Isabel to push the limits of her dreamwalking potential. She uses it once as an interrogation technique with somebody who isn’t dreaming, managing to catch a glimpse of an answer that the man didn’t want to give them, and several times tries dreamwalking to contact her brother Max at a distance, even when he isn’t dreaming, with mixed results.
Isabel is the only alien character on the show to be shown with the dreamwalking gift, though once a mysterious girl was able to pull Isabel into her dreamscape without her intent. When trying to solve the mystery, Isabel wondered if the girl was dreamwalking her, but when we learn more, it seems to be more likely that Isabel was subconsciously picking up some sort of emotional “distress call” and locking onto it with her dreamwalking powers.