In an IP telephony system the term "Softswitch" encompasses the computer processes that control the sessions, the medium (voice, video or messages) and services. In simple terms, the Softswitch separates the elements of the network (Hardware) from the control of the same (Software).
It consists of one or several machines (as you grow) that control VoIP traffic and even the gateways between traditional telephony and VoIP in which case they link both types of network and manage traffic. It also ensures that the signal is processed, depending on the type of media, which type of codec will be used in each session, or even transcoded from one codec to another. It is based on the SIP Protocol (Session Initiation Protocol) or Protocol H.323.