1:1 NAT maps one public IP address to one private IP address by specifying a /32 subnet. This means having an otherwise local network computer accessible from the Internet through the WAN interface of your m0n0wall device. From a security perspective this also means that all traffic arriving at the WAN interface is forwarded into your network to the designated internal server. Be sure that you have secured the internal server.
Additionally entire subnets can be passed through the NAT. This could be used for situations when multiple connected networks are using the same subnet, such as two sites using a 10.0.0.0/8 subnet.
Depending on the way your WAN connection is setup, you may also need proxy ARP.