When you first start Thaumcraft this will be your starting tab, finishing this will unlock all the other tabs such as Arcane Infusion, Alchemy and Auromancy.
This tab also holds hidden research that you will unlock as you scan more and more items and when you unlock other items in thaumcraft. Completing tabs and scanning items will sometimes unlock secret research.
Unlocking these generally only give you information about the item e.g. Pech research will give you more details about Pech. Most of these hidden items are not important in the progress of Thaumcraft but will help you “complete” Thaumcraft.