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Quoted from Virtual Moon Atlas glossary
Libration.
Apparent oscellatin of the Moon for a terrestrial observer. With its effect, it's possible to observe a small part of the Far Side. It exists 3 different librations: Longitude Libration, Latitude Libration and Diurnal Libration.
Longitude Libration.
According to constant rotation speed and to variable revolution speed of the moon, longitude libration allows a terrestrial observer to see East and West limits of the far side of the moon.
Latitude Libration.
Because of the Lunar orbit inclination compared to Earth orbit around the Sun, it's possible to see North and South limit parts of the Far Side of the Moon.
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