Welcome to TBT! You may wanna read the FAQ in The Bulletin Board subforum. That'd be your best bet on terms of learning about TBT and how you can spend it.
The forum bells (TBT) are basically used for the same thing in game bells are, buying and selling! They’re basically equivalent to the in game bells in that you can buy stuff with them (villagers, rare items...) but they’re worth more, I can’t remember the exchange rate so I’ll get back to you on that! And you get them by participating in events on the forums or selling stuff in your game through the forum