Stands for "Heat Sink and Fan." Nearly all computers have heat sinks, which help keep the CPU cool and prevent it from overheating. But sometimes the heat sink itself can become too hot. This can happen if the CPU is running at full capacity for an extended period of time or if the air surrounding the computer is simply too hot.
Therefore, a fan is often used in combination with the heat sink to keep both the CPU and heat sink at an acceptable temperature. This combination is creatively called a "heat sink and fan," or HSF. The fan moves cool air across the heat sink, pushing hot air away from the computer. Each CPU has a thermometer built in that keeps track of the processor's temperature. If the temperature becomes to hot, the fan or fans near the CPU may speed up to help cool the processor and heat sink.