Version control is a system that records changes you make to a file or set of files over time; this way, if you want to use a previous version of a file, you can recall that specific version.
Normally, once you save a version of a document, you cannot revert back to its previous version. You can only undo minor changes before saving the document, but once you save and exit the document, any previous versions are gone.
With version control, you can have access to multiple versions of a document, and you can have a record of all the changes that has been done to a document. Your file's version are stored in a version database, also called a repository. If you don't like a current version of a document and want to revert back to an older version, you have that option.