We already know all about the Skrulls in Captain Marvel, thanks in large part to the fact that they're heavily featured in the trailers that have come out so far. But those less familiar with the incredibly powerful heroine might not know that her story involves a second alien race: the Kree. If you're not aware, Captain Marvel is technically associated with the Kree alien race: in the comics, her father his human, and her mother is a Kree champion.
It's entirely possible that the backstory of Carol Danvers has been tweaked for the big-screen treatment, which means she may not have the same parents in the Marvel Cinematic Universe. Even so, it's clear her alliance in the film is with the Kree. Mar-Vell (played by Jude Law), who appears to have assumed a mentor-like role for the film, is a noted member of the Kree. Then there's Ronan the Accuser, who first appeared in Guardians of the Galaxy and will return in Captain Marvel, with Lee Pace reprising his role. More Kree appear in the photos we've seen from the film so far, and they're usually flanking the Captain herself.
There they are! Now that you've honed in on the Kree, so to speak, what's the deal with them? The primary trait of the Kree is their blue skin, though not every Kree has it. Some, like Jude Law's Mar-Vell, are born through interbreeding with humanoids of other worlds, resulting in a skin tone that looks closer to some actual humans. These individuals are called "Pink Kree." The Kree have been at war with the Skrulls for nearly a million years, thanks to a disastrous encounter early on in their respective developments.
The Kree went on to spread their gargantuan empire throughout the universe; the Kree Empire is made up of nearly every world in their galaxy. They're known for their extremely advanced technology and powerful military force. Technically, there is a point in the comics when the Kree attack Earth, which could mean that they're at odds with the Skrulls, who are obviously trying to invade in Captain Marvel.
That, in itself, seems to be the crux of the story: the Skrulls are trying to conquer planet Earth, and somehow, the long-standing conflict with the Kree comes into play. We just have one question: Where does that cute little Flerken come into the picture?!