The teaser trailer is a recording from a member of The Banished, a group of aliens that broke off from the Covenant, the main antagonists of the Halo series. The Banished feature the same types of aliens in the Covenant, such as Grunts, Hunters, and Elites, but don’t follow the same belief system.
The Banished were the main antagonists in Halo Wars 2 where they followed a Brute named Atriox. The Banished occupied the Ark in the main campaign and released the Flood at one point. The new ominous message seems to indicate some things have changed since their last appearance, however.
The message says The Banished occupy a Halo ring and that humanity will burn. The speaker also mentions following in “his legacy,” which might mean Atriox is dead or was replaced as their leader. He could also be the person recording the message, but the wording seems to indicate one of the former options.
Wiping out humanity would be simple with a Halo ring, a massive space weapon, so this will likely be a major part of the Halo Infinite plot. The teaser has given fans a little more information but has also left them with more questions.