Nirvana is a place of perfect peace and happiness, like heaven. In Hinduism and Buddhism, nirvana is the highest state that someone can attain, a state of enlightenment, meaning a person's individual desires and suffering go away.
The origin of the word nirvana relates to religious enlightenment; it comes from the Sanskrit meaning "extinction, disappearance" of the individual to the universal. Achieving nirvana is to make earthly feelings like suffering and desire disappear. It's often used casually to mean any place of happiness, like if you love chocolate, going to Hershey's Park would be nirvana. On the other hand, if you're a Buddhist monk, it may take you years of meditating to reach nirvana.