Before we get into it, it's important to note that not everyone will be able to take advantage of this feature.
For starters, you'll need a television or monitor capable of outputting 120Hz (sometimes advertised as 100Hz by retailers in Europe - ask if you're unsure!). If you then also want to play games at 4K resolution as well as 120 frames per second (where supported), you'll require a TV or monitor with an HDMI 2.1 port.