Siege of Dragonspear is an "interquel", taking place between Baldur's Gate and Baldur's Gate II: Shadows of Amn. The new game will take approximately 25 hours to complete and will use the same Infinity Engine as the old games. It is officially an expansion to Baldur's Gate: Enhanced Edition (despite being almost as large as the entire original game), so you will require that to play it.
This isn't Baldur's Gate III but more of an extra episode in the saga, filling in the blanks on what happened between the first two games. There wasn't much of a narrative gap between the two games, beyond murky mentions that something happened that encouraged you to leave Baldur's Gate after you'd saved it in the first game.
The new game has some interesting pedigree: the developers have avoided the "evil for evil's sake" villains of the original game and even got the much-in-demand Chris Avellone on board to review the script. There's also been updates to the graphics, UI and inventory systems which will roll out across all of the other Infinity Engine Enhanced Editions as well.
Apparently Beamdog want to ultimately combine Baldur's Gate, Tales of the Sword Coast, Siege of Dragonspear, Baldur's Gate II: Shadows of Amn and Throne of Bhaal into one humungous mega-game that will take hundreds of hours to complete and take players from Level 1 to (at least) 27. But that project is apparently still a way off in the future.