A thermal paste is that material that you place in between the upper surface of the CPU and the lower surface of the heatsink. It’s primarily used to fill the microscopic gaps between the two surfaces that impedes a proper heat transfer between the two surfaces. Without it, the heat-transfer process won’t be as effective as it should be, and the CPU will fail to dispose of the high temperature it generates during operation time, that will eventually lead to malfunctioning or even serious defects to the micro electronic circuits of the CPU causing it going out of action.
This video explains this situation briefly in a simple style:
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