It's essentially a new port co developed by Intel and Apple (although it's also available on PCs too) which uses a single USB C connector to carry power, USB, DisplayPort and, most importantly, PCI-E over a single cable. This allows connection of regular USB peripherals and monitors all the way thorough to PCI Express devices such as full graphics cards. With my system I use a ThunderBolt 3 docking station (CalDigit TS3 Plus) connected to all my devices so with a single cable into my laptop I can connect power, a pair of monitors, keyboard, mouse, audio interface connected to my speakers and a gigabit wired network. If I wanted to I could even daisy chain a GPU off of the dock and still have a single cable to the laptop. It's also fully backwards compatible so you can still connect regular USB and displayport devices. It's by far the most flexible interface available on the market.