Not all Coda docs are used in the same way. You'll inevitably have a few that you use every week, and some that you'll only use once. This is where starred docs can help you stay organized.
To star a doc, hover over its name and click the star icon:
Once you star a doc, you can access it quickly from the Starred tab:
And, as your doc needs change, simply click the star again to un-star the doc.