That's where your copyrights are filed. A not so well understood fact is that You Own Your Copyrights from the time you create and affix your work to any medium. The copyright office is a government facility which registers your copyrights. Nothing more, nothing less. But don't let that statement make you think registering your copyrights is unimportant.
Registration is extremely important.
It is your government record defining your date of creation, without which you have vary little chance of winning any legal dispute. Registration is your legal receipt of recordation of rights. At he copyright office all copyright forms are free and filing fees are vary inexpensive.