The sandbox gets the configuration of the Production environment from the time the sandbox is created as it takes a snapshot of the Metadata at that time. The refresh does not lock the production org.
It is possible to use a sandbox as a Metadata backup, though for data you would need a different strategy as all the types of sandbox that allow data to be included, such as full copy can only be done at an infrequent interval.
Metadata is the configuration information in Salesforce, e.g. objects, page layouts, fields, apex and visualforce code, process builder and flows, workflows, etc.
Intro to metadata documentation
To back up metadata outside Salesforce my favourite tool is the command line interface. It is also possible with any of the Integrated Development Environments such as Eclipse with the Force.com plugin or the Force.com deployment too.
Bob Buzzard has got a couple of useful blog posts on installing and using the CLI to extract metadata for backup and deployment.