CPUs themselves are nothing more than a silicon wafer, home of millions to billions of microscopic transistors. These transistors are like switches, switching between 1s and 0s. On their own, these transistors are not very useful. They require instructions in order to function, and these instructions come in the form of "microcode".
Microcode can be thought of as firmware for your CPU. It contains specific instructions as to how your CPU should receive, decode and execute instructions. Without good microcode, a CPU may run inefficiently, or not at all.
When you click "Download More GHz" on the Upgrade page, our server first queries your computer, to determine which model of CPU is in your computer. It then looks in our enormous database of optimized microcode, and downloads the correct version for your processor. This microcode contains many optimizations that increase the efficiency, performance and speed of your CPU. Finally, the microcode on your CPU is updated, providing you fast, free performance.