Wi-Fi Offload involves the switching of the radio access technology from the cellular network to the Wi-Fi network.
When the mobile device is not in Wi-Fi coverage, it is served by the macro-cellular network. As the mobile enters a Wi-Fi coverage area, the mobile device detects the presence of Wi-Fi.
Based on the operator policies and user settings, the mobile device is moved from the cellular network to the Wi-Fi network.
Because the cellular network does not carry data traffic for such mobile device, the network saves resources, which can be used to serve other macro-cellular users to exchange their performance.