Two types of messages are displayed in the echo line.
Informational messages tell you something useful, but do not imply in any
way that something is wrong. These messages are always enclosed in square
brackets ([ and ]).
Error messages indicate that something has
prevented a command from being executed. They are never enclosed in square
brackets, and will be accompanied by a beep from the terminal's bell if
the sound of the bell is not too obnoxious.
The echo line is normally cleared by the next keystroke. However, there is a special hack to clear the echo line without doing anything else. If you type a Space and there is a message in the echo line, then the echo line is cleared, and the Space does not insert.
The echo line is also used for asking and answering questions. After the prompt, you can type any characters you like. The reply is always terminated by a Return. Before you commit to the reply, you can delete characters by typing Backspace or Rubout, and you can delete the entire line by typing Control-U. You can also abort the command in progress by typing Control-G. Command processors are written to ask all questions before doing nasty things, so that you will never do damage by aborting a command.