What does the first scene in Fantastic Beasts and Where to Find Them mean?

This scene offers us our first glimpse at the person who will act as the "big bad" of the Fantastic Beasts franchise - Gellert Grindelwald, played by Johnny Depp. The scene you are talking about is intercut with newspaper headings talking about Grindelwald's rise to power and the fact that no one knows where he is - this is our first clue that the man in question is Grindelwald.

Below is another still from the scene you are asking about, showing us the back of the head of the person who kills the wizards and witches pictured in the question (you should be able to see that they are standing in roughly the same place, facing opposite directions - look at the ornaments that line the path).

Although we don't see his face at this time, later;

We learn that Gellert Grindelwald has been operating in New York while disguised as Auror Percival Graves (Colin Farrell). When unmasked by Newt Scamander, we see that Grindelwald and the person pictured above are the same person thanks to their distinctive hair styles.

This offers us definitive proof that the scene at the start was showing us Gellert Grindelwald killing a bunch of wizards and witches, presumably Auror's who had been tasked with finding and arresting him.

As such, the purpose of this scene was to a) introduce to us the villain of the film and b) show us just how dangerous he is.

It's probably worth noting that this isn't the first time we've seen or heard of Gellert Grindelwald. He has strong links to the story told throughout the Harry Potter books and films, having been a friend of Albus Dumbledore who later turned to the Dark Arts in his quest to unite the Deathly Hallows, forcing Dumbledore to duel and defeat him in 1945.

Coincidentally (or most likely not), J.K. Rowling has confirmed that the final film in the Fantastic Beasts franchise will be set in 1945;

Our story starts in 1926 and ends in 1945. It's just too big to fit in one movie!

It seems to be very likely that we will see a younger version of Albus Dumbledore introduced in this franchise, who will most likely duel Gellert Grindelwald in the final movie.

This film was set a long time before the Harry Potter franchise begun and so, as far as we can tell at this point, this really is mostly just the filmmakers seizing an opportunity to introduce this new story with a dramatic, explosive beginning. It also proposes a mystery, of which more is uncovered as the story develops.

It's only a moment, but to my memory there is a large, dark house, being approached by several official-looking men (MACUSA I imagine), and a white-haired man defeating them all quickly and effortlessly. In movie-language, this effectively presents us with our main villain, dispelling any curiosities from the very beginning that there may be some involvement by Voldemort, Tom Riddle, or other familiar character from the Potterverse.

As this film was always intended to be the first of a series of films, there are several unanswered questions. One being the imagery of this introductory scene (which in its nature almost feels like a flashback, or in the HP-world, possibly a vision or premonition), but also the Lestrange lineage mentioned briefly. There is plenty to uncover throughout this series of films presented initially within this movie, and I believe most characters met in this film will feature again in future to explore their stories and character arcs.

I firmly believe that this introductory scene will best be played out as a slow-burn mystery, only revealing the meaning of the scene in the last film of the franchise. The Harry Potter films have found huge viral success and has sustauined its longevity by having so many small, tiny, seemingly insignificant details uncovered for years after its release. To embark on a new movie franchise, appealing to the same fans, and set in the same universe, without featuring hugely overarcing mysteries and secrets, would be a huge waste of a long-set up ideology.