The equipment set overview shows all your equipment sets. An equipment set is a collection of related equipment (for instance custom ruby helmet, chestplate, leggings, and boots). You can configure bonuses (for instance extra armor or health) when a player (or mob) equips multiple pieces of the equipment set (for instance both a ruby chestplate and helmet). This menu should look like this:

There are 3 elements on the left of this menu:

Furthermore, there are 3 buttons for each equipment set you have made: