Well, simply, the JDBC driver is the implementation that know how to get to the particular database.
For example, let's say you want to use JoeBob Database (tm). First step, obviously, is to setup the database. Second step, get the JoeBob database implementation driver from Joe Bob corporation. This will most likely be a jar file, that you will need to add to the class path.
And finally, if you are using an old implementation, you will need to add a line of code that will register with the JDBC manager, on how to find the drivers to get to JoeBob database (hence, your question). Also note, with newer versions of Java, there is an option in the jar file that will automatically load the driver, so that line would no longer be needed.