What does it mean when an octet in a subnet mask isn't a full 255?

Say we have subnet mask 255.255.192.0 with IP address 192.168.1.37.

I know that when it's three full octets like 255.255.255.0 that the usable IP addresses start after the third octet because anything in the first three is locked by the mask. Very hard for me to accurately describe what is going on, but you get the idea. The first three octex being 255 mean that the first three octet of the IP address are never going to change, and that the last octet will be the host addresses. But I don't understand how this changes when the final architect is not 255 but instead something like 192. Can someone help?