Bravely Second is a direct sequel in the same world with the same characters. Bravely Default 2 is a new game in the same franchise, not connected to the previous games.
Oh, okay so I don't need to play the old ones then. Good to know I can just start the new one. Now to wait for price drop since I don't trust the gameplay balance. o_o
btw i feel this a good time to remind you guys that sea of thieves was made by rare, who also made DKC1-3 and DK64, alongside banjo kazooie and conker's bad fur day