SharePoint provides OOTB support for document versioning, allowing user to check-in/check-out files, and keep history of changed documents. But unfortunately, versioning system doesn't provide inbuilt method to find these all check-out items made by users.
For some administrative task and farm maintenance it's recommended to have all files to be check-in before making changes. And this task became daunting task for administrators, because you need to check all site collection and list collections manually via SharePoint interface , to check-in user files.
The way to get the list of all check-out files across specific site is to use the following SQL script for SharePoint AdminContent DB
Take into accoun that using SharePoint database directly is not recommended approach.