You are about to visit Agere Systems AC'97 Modem
"HP Customer Support | HP® Customer Support"
The Agere Systems Ac97 is a 56kbps modem driver, usually found in desktops and laptops that were made at the turn of the century (some modern units still use this, but are entirely optional - laptops rarely have this piece of hardware anymore due to space and hardware limitations). Such modems were extremely ubiquitous in the late 90s and the early 2000s, and featured a connection process that involves using an existent phone hard line to connect to a remote gateway (and subsequently the Internet).
Intel engineered the AC97 as an audio codec, which is designed to both provide audio functionality and modem capability. It was so-called as it was the standard that shipped with Intel chipsets in 1997 and was also called MC97, or Modem Codec 97. This was later replaced by Intel's HD Audio standard which boasted audio output in channels more than Ac97's 6, although theoretically standard chipsets only offer up to 8 sound channels.
As an overview,drivers are small pieces of software that usually cannot be modified by the end user, to allow communication between different hardware devices. In other words, a driver is a software buffer between devices, and must therefore be present in order to fully use and utilize the capabilities and functions of the device. The Mobile USB Modem software in this case is used so the computer can recognize your phone and use it as part of its hardware family.