Wi-Fi 6, aka 802.11ax, delivers faster speeds, greater network capacity, improved power efficiency, lower latency, and connectivity improvements in areas with several Wi-Fi devices. Wi-Fi 6 devices also must support WPA3, the latest version of the Wi-Fi security protocol with improved cryptographic strength.
The upcoming iPhone 11, iPhone 11 Pro, and iPhone 11 Pro Max all support Wi-Fi 6, and the devices will likely be certified under this program soon. More information is available on the Wi-Fi Alliance website. CNET also has a good explainer.