IVR stands for Interactive Voice Response. It is a technology that enables an interaction between a computer and a human.
For example, when a person calls, the IVR greets the caller with a welcome message. The caller is then required to choose from a predefined set of options such as press 1 for sales, press 2 for support and so on. After the choice is made, the call is routed to the chosen department/agent. If none of your agents answer the call, it automatically gets sent to a voice mail.