A "draft" is saved content for an email invitation. When you fill in a block of content (e.g., introduction, promo description, watermarking disclaimer or footer), you have the option of saving that content as a named draft.
Why would I save an email invitation draft?
If you plan on re-sending an invitation and don't want to have to copy/paste or re-type content into the invitation, you should save a draft the first time. The next time around, select your saved draft and the content automatically loads.
You might have a common introduction you like to add to all of your invitations. In this scenario, type your custom introduction and leave all other fields blank. Save the page as a draft. Next time you send out an invitation, select your introduction draft, and it will load your common introduction onto the screen.