There are a number of statuses that pages and page modules can be in. Here is an outline of each of them:
Unpublished - Content that is unpublished will not appear on the live website. As well, any content that is unpublished will not be available to 'Preview'.
Staging (content not yet published) - Content that is in Staging refers to content that can be viewed in 'Preview' mode but not on the Live website. This is useful for when you are making content changes and would like to preview the changes before publishing them live.
Published with Pending Changes - A solid green icon indicates that the current page or module is published, however, there is content that has been updated that has not yet been published. This content will be viewable in Preview.
Published - A solid green circle with a checkmark in the middle indicates that all of your content on the page is on the latest version