A neutron star is the incredibly compact core that remains after a supernova event.
When a high-mass star comes to the end of its lifetime, its outer layers collapse onto the core, compressing material to the point where the atoms are smashed apart, leaving only neutrons - sub-atomic particles with no electric charge.
The outer layers are then ejected in a super-massive explosion, leaving a rapidly spinning neutron star behind. Some neutron stars have been found to rotate at several hundred times a second.
A neutron star can weigh the same as one or two Suns and yet will only be about 20 km across. For comparison, a matchbox of neutron star material would weigh the same as the country of Wales. A house of it would weigh the same as the Moon.