A Supernova is a massive explosion which releases huge amounts of energy from a star, and then fades out over the course of a few weeks. I is caused when a 'White Dwarf' collapses into a neutron star , but Type II is caused by the violent death of a giant star with at least ten times the mass of the Sun. Massive stars can carry on burning fuel for longer than Sun-like ones. When stars like the Sun exhaust the hydrogen at their core, they usually have enough energy to burn helium, but, once this is exhausted, they puff off their outer layers, and dwindle away to a slowly-fading White Dwarf.
More massive stars can burn the heavier elements produced by the helium burning stage, and so on.
There is a catch, though - the more massive a star is, the more rapidly it uses up its fuel, and each new burning phase is shorter than the last one. Also, generating energy by nuclear fusion in this way relies on the energy of the product being less than the total energy required to make it, so that the excess energy is released in an 'exothermic' reaction. The reactions to form the heavier elements become less and less exothermic - up to the production of iron. So when the star attempts to burn iron, it suddenly shuts off generating energy at the core, and starts absorbing it instead. The continuous production of energy at the core is the only thing that stops the star from collapsing under its own weight, and so this is exactly what it does, with the outer layers rapidly falling in onto the core.