Try these at the command prompt:
man mount
man fstab
man umask
Basically, the 'man' stands for manual pages, which are Unix/Linux's help system. There is another called 'info' that also includes a number of additional commands and instructions. Just type 'info' at the command line to use it. 'q' exits both.
More information is online at
www.tldp.org as well.
Oh, and 'apropos
keyword' can help you find what man page you are looking for.