Within Buddhism, there are different tenet systems that have slightly different presentations or interpretations of the subtle meaning of liberation and nirvana, which requires a longer discussion.
The short answer: nirvana is the state of having eliminated all our disturbing emotions and distorted attitudes so that they can never arise in us again. The long answer...
Today is the day many Buddhist celebrate Nirvana Day (sometimes referred to as Parinirvana Day). This holiday is the day many Buddhists observe the death of Siddhartha Gautama, and consequently his transition into Nirvana.