That whole screen is for the WAN settings. At the top it says "On this screen we will configure the Wide Area Network information".
For 99.9% of installs MAC Address, MTU and MSS are left blank - you should leave them blank unless you have some really special reason to need a magic value.
For now, if you are leaving your front-end router/modem in its modem+routing mode, then you can leave Selected Type as "DHCP". It will be given a private address and default gateway from the front-end device that is in "front-end LAN = pfSense WAN" and use that to get out to the internet. It will be just as if the whole pfSense box, and the "pfSense LAN" behind it are 1 client on the front-end LAN. In this configuration the pfSense LAN will NAT onto front-end LAN, then front-end LAN is NATed out to the real internet.
At a later stage you can put your modem/router into bridge mode (so it stops doing "router") and passes the real public IP through to pfSense WAN.