In order to memorize a command, it is usually a good idea to learn where it's name comes from. For example,
cp comes from copy.
usermod means "User Modifiy" (at least I guess so).
Where does chmod come from, what does it mean?
It is the full form of the command. So basically you are changing the mode set as something to some other thing.
Read only permission to Read/Write permission, revoking read/write permission to just read only permission etc.