A T-bar piercing is a piercing with three or more exit holes and a single piece of jewelry. The jewelry "branches" inside the body. The barbell is most commonly in the shape of a "T", but it also be in the shape of an X, or ring with a barbell that screws into it at a ninety degree angle.
Generally a normal straight-line (two-exit hole) piercing is done and healed. Then, a third hole is made and the custom jewelry is inserted.