If i'm not mistaken its like the umask command in linux.
Basically it tells the system with which permissions to create new files. For example, in linux for each file there is a number (several actually) that show the permission of the owner, group and everyone else to read/write and execute the file. In numbers it is: read=4 write=2 and exec=1
So the number 7 indicates you can read write and exec the file. The number 5 means you can only read and exec and you get the idea...
The umask is the minus number that gets applied to all new files. So if usally a file is created with 666 permissions (everybody can read and write to it) then a umask of 022 means that the real number it gets created with is 644 - so only you can read/write and the rest can only read.
Hope it made sense and that it is correct for AIX as well.