A futurist is someone who has an interest in or studies the possible futures of our world, and who generates work related to their predictions of the same. It's a pretty broad term, and would encompass certain academics, as well as writers and other enthusiasts. Their basis in evidentiary processes is obviously going to vary greatly, by it's not necessarily accurate to say that they are simply guessing. It's not a licensed term, but neither is "author" or "crypto-bro" or "politico" and yet one can certainly be any of those without any credentials.
There are certainly some elements of guesswork and hypothesis in any predictive field. Certainly there will be authors out there who might style themselves as futurists, but whose works are much closer to fantasy than speculative science fiction. I (personally) think it would be grossly inaccurate to call Star Wars a futurist work, for example. But I think one can easily look at the history of technological and social developments and create something that is a reasonable prediction of the future. I think Weir does a respectable job of portraying a mostly-plausible scenario in The Martian, by comparison. Asimov's work has various examples that could probably be called Futurist, as well.
Why might a director or script writer consult with such individuals? Well, partially I suspect it's to claim that they did. It's a headline and marketing thing, of course. But also, because they're making a piece of media that's not necessarily in an area that they're intimately familiar with. Even if you've made several science fiction movies before, I think it's probably obvious that taking advice from someone who has spent thousands of hours considering the minor nuances and smaller details of something would have some useful insights.