Archived questions keep the data that has been collected for reporting purposes, but are not displayed on the public-facing project site.
To archive a question, click the "file box" icon on the question.
Once you've archived the question, it will be condensed, and appear at the bottom of the list of active questions.
You also have the option to archive the question from within the question's settings:
Click the Options drop down menu
Click Archive Question (show as past question)