Option 66 is a DHCP option, configured in your DHCP server (usually the router), that, upon obtaining an IP address, devices set with Option 66 enabled will also receive the address of a provisioning server, where they will immediately attempt to autoprovision. This is most often used in conjunction with TFTP, rather than HTTP.
Yealink and IPitomy phones both by default have Option 66 enabled, and other phones may have it either enabled by default, or at least support it and require it be turned on.
Enabling Option 66 and pointing it at the IP address of the PBX allows you to take a fresh Yealink or IPitomy phone out of the box, plug it into the network, and, assuming you've configured the extension on the pbx with the phone's mac address and model type, the phone will automatically download its provisioning file and configure itself with no further hands-on at all. This can greatly speed up the installation of systems with large numbers of phones, and make it easier to factory default a phone with the intention of reprovisioning it, as the reprovision is automatic.