Electronic devices such as microphones, musical instruments and turntables emit a low-level signal. Because the signal level is low, the signal must be processed before it can be manipulated or transmitted through an amplifier and heard through speakers.
Each electronic device has what is called a nominal operating level, which is the device's standard signal strength for its intended use. The term "line level" refers to that signal strength expressed in decibels. A preamplifier is used to bring a weak signal up to the line level necessary to manipulate the signal so it can be heard.
A digital preamplifier is a preamplifier that not only processes analog signals, but can also process digital signals.