The Short Answer
MIL-STD-704 sets the electrical power interface for equipment on military aircraft. It covers the normal voltages and frequencies the aircraft supplies, and the transients and abnormal conditions, such as power interrupts and surges, that equipment must tolerate without damage or unacceptable disruption.
Why It Matters
Airborne computers and displays draw from a power bus that varies with engine, generator and load conditions. Designing to MIL-STD-704, or protecting the input to its requirements, keeps the system stable through those variations. It is the airborne counterpart to MIL-STD-1275 for ground vehicles.
See our overview of U.S. military standards (MIL-STDs) for how this fits alongside the others.