"Optional Site Wide Fields" are site wide questions that appear during the listing process for the seller to use while placing their item on your site. When used properly they assist your sellers in helping them provide clear and concise information about the item they are putting up for listing. The administration for these fields can be found here:

Listing Setup > Fields to Use

You have a variety of options in implementing each of these fields. You can choose whether or not to "use", "require", "display", add an "other" box, make it editable, etc. Also, similar to Category Questions, you can elect for it to display one of your Pre-Valued Dropdown boxes. Optional Site Wide Fields are also search page dynamic in that they will display on the Search Page as search criteria for your site visitors to use if desired.