An A/D converter converts an analog (electrical) signal into a digital signal (0's and 1's). In the case of audio, a line level signal is usally around couple of hundred millivolts (correct me if I'm wrong anyone). A computer doesn't know how to read this information being sent to it, as it doesn't undertsand the "language", because computers can only understand the digital "language". So to make the computer be able to understand what it's being it sent, the analog signal has to be converted into a digital signal - this way the computer can understand what it is being sent. Think of an A/D converter as a translator - your computer doesn't understand the "foreign language" it's being sent - so through an A/D converter - "the translator", it is able to make sense of the information being sent to it.
Hope this helps