The Bulletin Board is a simple notice board in the Chao Kindergarten lobby that you can interact with to read. On the Dreamcast game, this was used to connect to the internet with a built in web-browser to link you to the official Sonic Adventure 2 Homepage BBS (abbreviation for Bulletin Boards).
The Sega Dreamcast could connect to the internet via it’s 33.3k (!) modem, using a phone line. These message boards were actually the place where Chao Island was born along with many other fansites! These remained open even for the Nintendo Gamecube releases and were eventually shut down a few years after.