A neutron star is what’s left behind when a massive star (typically 8-30 times the size of our Sun) explodes into a supernova. These supergiant stars get so large that they are no longer able to remain stable under their own intense gravity, collapsing in on themselves.
The gravity is so massive that it exceeds the strength of the atomic forces within particles, causing them to eject protons and electrons. The ball of neutrons they leave behind is so dense that a teaspoonful of the material would weigh as much as Mount Everest!
Neutrons stars have a “mass threshold”- if they take on too much mass, even the neutrons themselves will collapse. When two of these extremely dense neutron stars collide, the extra mass they add to one another causes their massive gravitational forces to tear each other apart.
They go into a blindingly-fast death spin, ejecting massive amount of material while merging into a doughnut like structure with a black hole at its center. The entire process takes just 20 milliseconds (that is 1/50th of a second, if you’re wondering).
Check out a simulation of the amazing phenomenon courtesy of NASA: