Libration is the apparent side to side rocking and up and down nodding of the Moon. There are two components: the more obvious "optical" and the very subtle "physical" libration. (www3.telus.net)

Although the Moon always presents us with the same face towards the Earth, due to its rotation and revolution being tidally locked to the same period, the combined effect of all these different librations allows us over time to see some 59% of the Moon's surface. (Wertern Washington University)

Libration in longitude is a consequence of the Moon's orbit around Earth being somewhat eccentric, so that the Moon's rotation sometimes leads and sometimes lags its orbital position. (NationMaster.com)