The LSI USB 2.0 Soft Modem driver refers to the software needed for a modem that connects to a computer via USB. LSI (formerly Agere Systems) was a manufacturer of modem chips, and a "soft modem" typically refers to a modem that offloads most of its processing tasks to the computer's CPU, rather than performing them on the modem hardware itself.
A Soft Modem works by using the computer's software to handle signal processing, making the modem cheaper and smaller compared to hardware modems. The LSI USB 2.0 Soft Modem is likely an external USB device that enables dial-up connections to the internet or other telecommunications services.