Hello annxyi, Tbt is the forum's own currency. Tbt means "The Bell Tree (Bells)" and it are points you earn by posting.
When you look in the sidebar (the text under your avatar), under Join Date you can see 'posts' and 'bells'. In your case, it means you have earned 11 Tbt by posting a total of three times.
When you (or any other user) write a post, you are rewarded Tbt automatically, which means it can quickly add up if you post a lot!
The reason people want Tbt as payment, is that Tbt is used as payment itself, for AC:NL (or Pocket Camp, or other spin-off games) villagers, services (such as a weeding town) or items. I myself use Tbt for payment towards people who sell rare items, sometimes region exclusives, which are hard to obtain for me otherwise.
Also, Tbt can be used in the Shop in the forum. You can find the shop under the page's 'The Bell Tree Forums' text, to the right of "Forum home" and "Animal Crossing World". The shop is used for seasonal events (such as halloween or christmas) and sells collectibles
I hope this post will be helpful for you! If you have more questions about the site, please do ask them!
Also, welcome to the forums