It is used to display informative or error messages, and to read lines of input from you when that is necessary. Almost all of the commands available in the echo area are identical to their Emacs counterparts, so please refer to that documentation for greater depth of discussion on the concepts of editing a line of text. Moves backward a character. Moves to the start of the input line. Moves to the end of the input line.
Moves backward a word.
Deletes the character under the cursor. If completion is being read, C-g discards the text of the input line which does not match any completion. If the input line is empty, C-g aborts the calling function. Inserts the next character verbatim. This is how you can insert control characters into a search string, for example.