The "first sale" doctrine says that a person who buys a legally produced copyrighted work may "sell or otherwise dispose" of the work as he or she sees fit, subject to some important exceptions (Section 109a). "First sale" gives you the right to loan a legally purchased book or CD to your friend. Historically libraries have heavily relied on the first sale doctrine to lend books and other items to their patrons.
The first sale clause was enacted during a time when most copyrighted works were produced in physical formats that made such works difficult to reproduce on a large scale. Many protected works including books are now produced digitally, however, copyright owners have lobbied Congress for new laws that some feel may undermine the "first sale" doctrine.
Additionally, many publishers (most notably music publishers) are now creating works to include technologies that interfere with the "first sale" doctrine. Software companies also attempt to circumvent the first sale doctrine by characterizing the consumer purchase as a license rather than a sale.