This link here explains about the process of obtaining one. It is free and should only take a matter of a couple of days according to the site.
It is used for librarians to be able to look up books in a national database usually in the format of YYYYnnnnnn. Y is the year you received the number and n is the control number that is sequenced during that year. The control number resets at the beginning of each year.
Here is also an Excerpt from a page explaining someone's opinion on the LCCN and it's benefits:
The entire point of having a LCCN is so that libraries can find, and
therefore order, your book for their library collection. On the one
hand, there are @ 9,000 libraries in the USA and if they all ordered
my book, I would be very pleased.
On the other hand, if my book is in
the library, then I am not selling more copies even though I am
gaining more readers.
So why do I want the LCCN? Because I absolutely
love libraries and I want my book in the library because it’s part of
my vision of being a writer. A vision of a print book on a library
bookshelf – like all those thousands of books from the library shelves
throughout my life, since the day I could read.
But besides visions
and dreams, having my book in the Library of Congress database is part
of the magic of SEO and keyword searchability that is an ongoing, slow
process similar to the formation of a coral reef. One coral reeflet is
lost in the wide ocean, but over time, the reef can grow until it
gains the attention of everyone who passes. These small registry
actions I take will accrete over time and make my name more findable,
and my books more available.