During the original Battle for Azeroth reveal, Blizzard announced that one of the Allied Races the Horde will get on launch will be the Zaldari Trolls. However, as development on Battle for Azeroth progressed, the Zandalari Trolls were replaced with Mag'har Orcs and delayed for an undetermined amount of time along with the Kul Tiran Humans. Many expected they would arrive with the first content update, but alas, the Tides of Vengeance patch came and went without either of the two races becoming available.
Thankfully, the extremely long wait will soon be over as Blizzard has now finally announced that Kul Tiran Humans and Zandalari Trolls are ready to go. So if you're eager to start a brand new character, make sure to put a pin on March 12th as that's when both of the new races will become available.
Much like the previous Allied Races, you will need to do a couple of tasks in order to unlock the Kul Tiran Humans and Zandalari Trolls. It's nothing too complex, however, so don't worry about missing out on any of the fun. All you really need to do is gain Exalted status with your chosen faction, as well as complete both the main storyline and the war campaign. In other words, if you've been playing Battle for Azeroth longer than a couple of weeks, chances are you already have everything you need.
You can read a little bit more about the two races, as well as check out their racial abilities, by heading over to the official website.