A "subnet" is a sub-network that operates in a specified IP address range. For example:
192.168.0.1 to 192.168.0.254 is a subnet
192.168.1.1 to 192.168.1.254 is a different subnet
The Subnet Mask of 255.255.255.0 "masks" these two subnets so that they cannot communicate and can be used independently. If you change the Subnet Mask to 255.255.0.0 then addresses with a different third "octet" will communicate, i.e. the above two subnets will no longer be hidden from one another.
However, your router will operate in a particular single subnet and allocate IP addresses in that subnet, so I think you will be unable to configure it to use different rules for multiple subnets.