umask are the default permissions that are applied when a file or directory are created. To see this in action simply just "touch filename" or "mkdir somedir" and you'll see what default permissions are applied.
The first thing I always tell people you should know is to NEVER change the defaults unless you are making them more restrictive. But they work well and if you change the defaults you could end up creating a file without permission to read........
I have a directory structure which the primary user needs full access on but wanted a user to access only a specific directory within the main directory (so they could write in there only and nowhere else).
Here's what I did:
[root@compevo8001 dllnow]# ls -alh
drwxrwxr-x 2 someuser software 4.0K Oct 19 2017 ExpireYMD-2017-11-20
drwxrwxr-x 2 someuser software 4.0K Nov 19 2017 ExpireYMD-20........