A white dwarf is what remains after a star like the sun exhausts its nuclear fuel and sheds its outer layers. What is left behind is the hot, compact core, roughly the size of Earth but carrying a mass close to that of the entire original star.
A white dwarf no longer generates energy through fusion. Instead it is held up against gravity by electron degeneracy pressure, a quantum effect that keeps electrons from being squeezed any closer together, and it simply cools and dims over billions of years rather than collapsing further.
If a white dwarf sits in a binary system, it can pull in gas from its companion star and grow heavier over time. Should it approach the Chandrasekhar limit, that slow accumulation can end in a sudden thermonuclear explosion instead of a quiet fade. More detail is available from NASA’s Imagine the Universe.