Prequel or Backstory?

So who were the First Men? People who came to the planet from our world?

Wikipedia explains the Andals only briefly in this news of a prequel series:

"On June 8, 2018, HBO commissioned a pilot to a Game of Thrones prequel series from Goldman as showrunner and Martin as co-creator. The accepted prequel will take place in the Age of Heroes, a period that begins roughly 10,000 years before the events of Game of Thrones. Notable events of that period include the foundation of powerful Houses, the Long Night when the White Walkers first descended upon Westeros, and the Andal Invasion when the Andals invaded from Essos and conquered most of Westeros. Writing in a blog post in June 2018, Martin suggested The Long Night as a title for the upcoming series."