In terms of astronomy, a nova is when a star suddenly becomes a lot brighter, then gradually dims back to normal over several months. This can happen multiple times with the same star.
Novae happen in binary star systems where one of the two stars is a white dwarf. A white dwarf is a type of dead star. They have cores made of carbon and oxygen atoms that are packed as closely together as possible without violating the Pauli exclusion principle. This quantum mechanics law states that electrons can't share the same quantum state with other electrons - or, put another way, two electrons can't occupy the same space at the same time. The close packing of the electrons creates a pressure, called degenerate-electron pressure, that holds the star up against the strength of its gravity.
In a normal star, gravity is balanced by nuclear fusion reactions occurring in the star's core - the same ones that create a star's light. But a white dwarf, being dead, no longer has these reactions going on. Left on its own, a white dwarf slowly cools down and dims over time until its temperature reaches absolute zero.
When a white dwarf is in a binary system with a normal star or red giant star, however, hydrogen from the companion star can fall on the white dwarf's surface if they orbit each other closely enough. Temperatures rise steadily as the hydrogen accumulates, until it explosively ignites, fusing hydrogen into helium and creating the bright flash of light that we see back on Earth as a nova.