This is when the incoming power is regulated to ensure that the voltage and current needed are being provided. The electricity that comes in from the power company is not very "clean", and in fact, can vary from its claimed voltage by a significant percentage. (This does not even take into consideration spikes and surges from electrical storms.)

The load on a municipal power grid varies considerably, and the power provided ebbs and grows along with it. Add to that jags in the current caused by shorts in the system, sudden heavy loads, or the last game of the World Series going on during a heat wave. All of these things cause more or less than the rated voltage to come out of the wall plug.

Power conditioning involves running the incoming power through circuitry that stores enough energy to smooth out the peaks and troughs in the current while also filtering out spikes and electrical noise and at the same time modulating the voltage so that the device involved gets the voltage it needs to properly operate at all times.

Most electrical devices, like your household TV, have some power conditioning circuitry inside. But if you have ever observed noise in the picture during an electrical storm, or when someone is drying their hair, you know that it sometimes isn't enough. For critical applications like heart-lung machines and your email server, there is often a facility power conditioning center that monitors the power for the critical site, and sometimes the entire building. This is usually tied into the Uninterruptable Power Supply system (UPS) that protects it against blackouts.