A version control system allows you to track the history of a collection of files and includes the functionality to revert the collection of files to another version. Each version captures a snapshot of the files at a certain point in time. The collection of files is usually source code for a programming language but a typical version control system can put any type of file under version control.
The collection of files and their complete history are stored in a repository.
The process of creating different versions (snapshots) in the repository is depicted in the following graphic. Please note that this picture fits primarily to Git, other version control systems like CVS don't create snapshots but store file deltas.
These snapshots can be used to change your collection of files. You may, for example, revert the collection of files to a state from 2 days ago. Or you may switch between versions for experimental features.