"Baubles" is a small mod created by Azanor, which provides four extra equipment slots for special items. The extra slots can be seen and accessed by a secondary inventory command (B by default), or by clicking a small "ring" icon added to the usual inventory screen. There are three types of slots, each of which can only accept items designed for it: One "amulet" slot, two "ring" slots, and a "belt" slot.
In Thaumcraft, these will often be used for the "Runic Baubles" which provide Runic Shielding, but there are a few other options. Other mods may also provide items intended for these slots.