Man
The man command is short for manual. It allows you to read the instruction manual for almost every command in Linux and other Unix-like operating systems. Don’t know how to use an application? The first thing to do is to run the man command and then the name of the application.
Below is an excerpt of the manual for the man command itself.
> man man
Man: find all matching manual pages (set MAN_POSIXLY_CORRECT to avoid this) * man (1) man (7mp) man (1p) Man: What manual page do you want? Man: 1