The Microphone Preamplifier (aka Preamp for short), is a component of the audio recording chain responsible for increasing the overall level of an audio recording, captured by the microphone.
The microphone, due to it being an acoustic device, captures audio at near-inaudible levels and thus the preamp is needed to make the signal audible to the human ear.
However, not all preamps are made equal. Some introduce more noise and distortion to your signal during amplification than others. Be sure to inform yourself on Signal-To-Noise Ratios and check them before purchasing your next sound card or external microphone preamp.