I'm not familiar with AirVPN but do use pfsense. Resolver is a DNS server, while forwarder forwards to another server, such as an internal DNS server or ISP DNS server. Typically you would probably just want to set a DNS IP in your DHCP settings and use neither of those options. So if you use your ISPs DNS, Google, Open DNS etc you would just put that IP in the DHCP settings to be pushed to the clients.
The forwarder would basically do that, but it would use the gateway IP as the DNS server. Good if you want to hide the real server, such as if it's internal.