Commands used to be the way that all computers were operated. Since the beginning of keyboards and monitors, computers required users to provide text based instructions to do things like work on files or use applications. A command interface is often abbreviated to CLI (Command Line Interface or Interpreter). The commands you give are usually entered using a piece of software called a Terminal or Console. This software is usually a small tool such as the Gnome Terminal, xterm, rxvt or the Linux Console which traditionally runs the Bash (Bourne Again Shell) in Linux. The commands you give to the bash shell are interpreted to provide outcomes, for example the command 'chmod u=rwx file.txt' without the parenthesis would change the ownership permissions of the file called file.txt to be readable, writable and executable by the owner of the file (user).
Today of course, most computers are synonymous with the use of a Graphical User Interface or GUI, which uses Windows, Icons, Menus and Mouse pointers (WIMP) to operate the computer rather than commands.
The Gnome Terminal Application can be found under the Applications > Accessories menu in the Ubuntu Desktop.