it stands for bumping to the top.
You will see it on FSOT a lot. If you reply to your own thread it moves to the top of the page.
For a sure-fire humdinger of a thread start one on:The Duggars,Government Assistance, Politics, Crickets,
Religion,Circumcision, Infant Ear Piercing,Spanking,Breast Milk vs Formula,Feeding kids Fast Food/ Beans & Rice, Sniffing Butt, or Vaccinating.