The Sandbox is the place where users can get feedback on potential questions and answers. Apart from the Sandbox itself, this tag should be used for meta-questions about the Sandbox.
The Sandbox is a meta question which users can answer to get feedback from the community about questions they want to post but might have some issues. The Sandbox is the place for sorting those issues out so that when posted it is answerable as easily as possible.
There is also a separate Answer Sandbox where users can get similar feedback for answers they are thinking of posting but which are as yet incomplete or may have other issues.
Each Sandbox is slightly different; for full "operating guidelines", please see the Sandbox FAQ
The Sandbox was created from this question on meta.