A (subnet) mask is used to group IP ranges in a network.
To see, how it works, you need to translate the numbers to binary digits.
192.168.001.004 = 11000000 10100100 00000001 00000100
255.255.255.000 = 11111111 11111111 11111111 00000000
If you have another IP, translate it to binary, look at the positions where's a 1 in the mask. If all this digits are equal to the original IP, the IP belongs to the same subnet.
So in this case, all IPs of the form 192.168.001.x belong to the network.
Since 192.168.0.0/255.255.0.0 is reserved for private networks, no computer from outside will have such an IP.