Sub-GHz refers to operating frequency close to but not exceeding 1 GHz. For example commonly found 433 MHz, 868 MHz and 915 MHz which are listed as legal Sub-GHz ISM frequency band that allows Industrial uses, Scientific research and Medical operation.
Wireless communication has not only became major necessity today, rather it has been a "MUST" component in everyone's life. Start from radio, TV remote control, mobile phone, and now the hottest topic on earth "IoT Internet of Things".