For a general overview of nodelets, see What are Nodelets?.
Nodelets are specialized text boxes which contain important information and links, and which show up on every page.
Nodelet order is determined by the numbers you enter in the box next to each nodelet name. For those nodelets you'd prefer not to see at all, you can disable them by entering 0 in the sort order box.
You can have one set of nodelets appear on the FrontPage and a different set of nodelets everywhere else. Or you can choose to have the FrontPage nodelet settings apply to all pages.
All nodelets off: Nodelets will never be displayed. This takes precedence over all other nodelet configuration settings.
Use my frontpage nodelets everywhere: the Non-Frontpage Nodelets list will be ignored, and your Frontpage Nodelets settings will be applied to all pages you see.