A voltage regulator module (VRM) is an installable module that senses a computer's microprocessor voltage requirements and ensures that the correct voltage is maintained. If you are changing your computer's microprocessor (for example, changing from a Pentium to a Pentium Pro or a Pentium with MMX ), you need to add a voltage regulator module to the existing voltage regulator in the motherboard so that the new voltage requirements can be detected and accommodated.
In the case of the Pentium, the original Pentium has the same voltage requirement for its core or basic operation as for its I/O operation. Both use 2.8 volts. However, the Pentium Pro and the Pentium with MMX have different voltage requirements for core (2.8 volts) and I/O (3.3 volts). Adding a VRM allows it to regulate the voltage for I/O while the original regulator built into the motherboard continues to regulate core voltage.