The subnet mask is used in conjunction with the network address to determine which part of the address is the network address and which part is the host address.
A subnet mask may look like 255.255.255.0 which means the first 3 bytes are part of the network and the last byte is the address of the host on the subnet.
Another way to write the same network mask as above is /24 which means the 24 first bits (8 bits * 3 bytes) are part of the network.