The 510 adapter turns your standard 510 thread into a full sized eGo thread so that you can use a Wicked E-Tech BVC Clearomizer or Aspire K1 Glassomizer etc. (or anything with a female thread) with your new 40w Box Mod.
This adapter also has a plate which stops the adapter from threading too deeply into your atomizer which makes it much easier to disassemble.