An situation when the temperature inside reactor core reaches such
high levels that the fuel rods (made from Uranium) begin to
melt their way down and destroy the reactor.
It usually happens when fuel rods cannot be cooled by a
cold substance (for example heavy water) and they start vaporising every
single molecule of water nearby, thus accelerating the process of building heat. The only thing operators can do now to stop it is to
drop control rods, but it can be impossible because of built up pressure (that's how Chernobyl happened). When the control rods are down, the nuclear power plant crew still needs to resume water delivery to the reactor because of nuclear decay
TL:DR Nuclear fuel gets
hot and burn it's way down the reactor.
Guy from Fukushima: The
tsunami shut down our water pumps and now now the
heat level is over the critical point. We have to evacuate whole wing of power
plant and get fireman over here. Tell them there's high risk of Nuclear meltdown.