The power supplied by a preamp to a mic, known as "phantom" power, is DC current used by the mic to charge its diaphragm. Phantom power is usually delivered through an XLR connection, which also returns the audio from the mic to the preamp. Dynamic and ribbon microphones don't use phantom power to drive their diaphragms, but can be connected to the same XLR inputs without harm even with phantom power switched on. Likewise, some condenser mics use a battery instead of the phantom power supply.
The general phantom power specification includes 48 volts, but some preamps and mixing consoles supply less than 48 volts of power. Fortunately, microphones are generally built to accept a wide range of voltage below that, and will function properly with as little as nine volts. However, the current may also vary from 10 milliamps to as low as one or two. Higher-quality microphones may need more current to operate at their highest quality.
When you use a high-quality microphone with a preamp that offers low voltage or current, the mic may not operate at full efficiency, resulting in a quieter sound or coloration. While some manufacturers, such as microphone maker MXL, claim voltage can make a difference, others such as component manufacturer Rane point out that many microphones only use approximately five volts to power the diaphragm.
When you look for a microphone preamp, your primary guide should be your ears. If the microphone and preamp combination sound good, the phantom power voltage and current aren't as important. However, if you plan to expand your microphone selection, especially with higher-quality mics, the preamp you buy should offer power at close to the specified standard.