Each year, authors, publisher, librarians, and bookish people leave out ARCs or books of teen interest. They hope that teens will discover these great books. I've been participating at the library since the events starting in 2007.
Here's some history of the day from the Readergirlz site:
"In 2008 and 2009, readergirlz, Guys Lit Wire, and YALSA orchestrated publishers' donations of nearly 20,000 new young adult books to hospitalized teens across the country.
For 2010, If I Can Read I Can Do Anything joined forces with these three organizations to drop over 10,000 new YA books, donated by publishers, into the hands of teens on Native American tribal lands.
Each year, nationwide, librarians, hundreds of YA authors, and teens dropped YA books in their own communities to raise awareness for Operation TBD and Support Teen Literature Day.
Here are our logos and videos from former years. Enjoy!"