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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.
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