The checked out/in icon, represented by a lightbulb, reflects the state of a page or file. When you edit a page by clicking on an editable region button, or click to a page from a list view, the page is automatically checked out to you. When you publish a page it is automatically checked back in.
When a page or file is checked out to a user, no other users can enter the pages and make changes until the file has been checked back in. The Web Team can override this and manually check pages back in that are checked out, and doing so will cancel any unsaved changes to the page.
Status icons indicate the check in state of the files in the CMS
A lit light bulb indicates a page is checked out to you.
A red lock indicates the page is checked out to another user.
An unlit light bulb indicates the file is checked in and available to be checked out.