By default, the processor in the Raspberry Pi runs at 700MHz but it can be overclocked. Microprocessors are designed in such a way that they perform one unit of work per clock cycle. One unit of work could be adding two numbers together or fetching something from memory. The faster the clock frequency, the higher the performance. Overclocking means to increase the frequency at which the processor runs. The problem is that each model of microprocessor is designed to run at its default frequency and overclocking takes the processor outside of its nominal design limits. If overclocked too much, the CPU becomes unstable which results in crashes or even SD card corruptions. However modern manufacturing processes mean that most chips, including the Broadcom BCM2835 used in the Pi, can safely run at higher speeds.

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How to Overclock Your Raspberry Pi (Make Tech Easier)