When an upgrade-card or system vendor installs the CPU, it adds a clock chip, which sets how fast the processor actually runs. Speed ratings, however, aren't inviolable limits. When you install one of these upgrade cards, you can slowly increase the chip speed until your system starts crashing and then bring the speed down from there just a bit. The goal is to eke out every bit of performance that your particular CPU is capable of. In theory, it's as easy as slipping on an Acme jetpack and turning up the fuel flow, but in practice, tweaking your CPU's speed is a great way to blow up all your data.
Processors are complicated, delicate components.
Although a chip designer may decide that it's OK to bump up the clock rate on a processor, the decision is based on many years of experience and backed up by expensive test equipment that can tell if the chip is behaving correctly. When you push a processor too hard, you won't necessarily know when it starts producing errors. Eventually, these mistakes can add up, and your system may become unstable or simply refuse to start. Overdriven chips are also more sensitive to environmental conditions. If you're using your upgraded machine strictly for entertainment, you may be comfortable pushing it as hard as possible.