It is set two years after the last game. Breath of the Wild was a launch game for Switch that came out on March 3, 2017. It has sold over 14 million copies.
The debut trailer noted only that the game is development while showing Link and Zelda exploring some ruins. The game could be some time away, but this was a big surprise announcement for Nintendo to end its show on.
It is unusual for Zelda games to get a direct sequel, but the popularity of Breath of the Wild seems to have encouraged Nintendo into this direction.
You can watch the reveal trailer above.