Your Wi-Fi (or wireless network) uses radio signals to relay data between your devices and the Internet. It's set up using a modem that acts as an Access Point so your devices can send and receive data.
These are on a sticker on the back or bottom of your modem. Your Wi-Fi is set up to use those details straight out of the box. Make a note of them somewhere so that you don't have to look at the back of your modem each time!