Prime95 is a program which searches for very large prime numbers. It has become extremely popular among PC enthusiasts and overclockers as a stability testing utility.
It includes a "Torture Test" mode designed specifically for testing PC subsystems for errors in order to help ensure the correct operation of Prime95 on that system, but can also be used to stress test a PC. People who are overclocking their system may use the torture test as an indication of the stability of their system.
If your computer can run Prime95 for 24 hours without any errors, then you can usually assume that your CPU, and the parts of the motherboard which interface with the CPU and RAM, are stable and working well.