A "sandbox" is a wcm concept used to isolate one set of changes from the other assets in a web project.
An "author" sandbox is where a particular author's changes go. Typically an author will work on a set of assets and then send them to the next step in a wcm process.
The "staging" sandbox is where approved content from all sources comes together prior to deployment to the live site.
To complete the picure there are also "workflow" sandbox which contains the assets of a wcm workflow.
You can also have other sandboxes types for example to contain files from data feeds.
