It was a characteristic of certain machine guns, especially those firing from a closed bolt, that are air-cooled, and capable of sustained use. When the trigger is released the weapon feed leaves a final round into the chamber, the heat ignites the propellant and the round is fired.
In the case of the US M296 or other squad assault weapons sustained fire of only a hundred rounds a minute can create sufficient heat, without efficient cooling of the barrel or the chamber it is easy to built up heat to cause either a cook off or stoppage of the gun through metal expansion. With the danger of a cook off, a jammed gun with a hot barrel can need a wait of 10-15 minutes before the jam can be safely cleared.
The firing of rounds induced by other heat sources is also sometimes termed a cook off.