"Baubles" is a small mod created by Azanor, which is automatically downloaded and installed by Thaumcraft.
The mod provides four extra equipment slots for special items. These extra slots can be seen in the "Baubles menu", reached by an alternate inventory command (B by default), or by clicking a small "ring" icon added to the usual inventory screen. There are four slots of three types, each of which can only accept items designed for it: From top to bottom, one "amulet" slot (#1), two "ring" slots (#2 and 3), and a "belt" slot (#4).
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.
Items in these slots don't change your appearance.