A Joomla quickstart is a prepared installation bundle for deploying a site with a template, extensions and sample content.
How it differs from a template
A normal template package is installed into an existing Joomla site. A quickstart normally deploys a separate site with preconfigured menus, modules and articles. Its format and restoration process depend on the provider.
Using a quickstart
Install it in a separate environment following the supplied documentation. Check the source, Joomla and extension versions, PHP requirements and licences. Replace demonstration data, credentials and mail settings, then apply necessary updates.
A quickstart helps reproduce a Joomla demo site. Applying one over an existing production site without a supported procedure can cause data loss.