TBT stands for The Bell Tree (bells). It's also sometimes called TBTB and BTB. The abbreviations all mean the same thing. These bells are the currency on this forum. You obtain this currency by posting and making threads/polls. A good way to boost up your currency is to sell goods and services to other members on the site. This can include selling villagers, selling art work, in-game items, and even doing chores like weeding someone's town. The reason why people are using TBT to sell villagers instead of IGB is because of the TBT to IGB ratio. The ratio, last time I checked, is something like 100 TBT for every 15~20 million IGB. If you buy a villager that's worth say 500 TBT, and you wanted to pay with IGB, you'd have to drop 100 million bells in their town. That would take ages to do and would be quite a bit of labor. It's much more practical to use site currency than game currency. That ratio is like this as a result of a game that is now two years old. The game currency is severely inflated. Hopefully that answered your question.