Source control is one of those things that one rarely notices they need until it is far too late. Usually when you accidentally delete part of your source tree instead of simply moving it to a different location, or when you make a set of complex changes that leave you worse off than were you started (but you cannot go back). A source control system solves these problems by keeping copies of each revision of a set of files on a server, while giving you access to a local copy of those files to make changes on.
Good source control systems allow multiple people to modify a single file at once. Then they will try automatically merging changes between differing sets of modifications. A good source control system will also let you browse the history of a file or set of files (allowing you to "go back in time") and allow you to have access to your code from as many systems as you like. Subversion is one of those good source control systems.