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It basically means that your system overheated and shut itself down so as not to damage any components.
Reasons for this range from dust on the heatsink or a fan not spinning to heatsink not being seated correctly.
What is the make/specifications of the system and roughly how old is it?
If it began suddenly, maybe the heatsink has gotten jarred loose from the cpu. It'll overheat instantly without the sink. If this is the case, you should apply some fresh heat-conducting gook between the two.