This key enables and disables character insert. The condition of the function is show on the top line, e.g.
When turned off, characters entered overtype existing text.
When turned on, new text is inserted before the cursor position. This is useful for adding characters to the middle of a line.
An exception to the above rule is carriage return (Enter). With insert on, a carriage return is added as with any other character, causing a break of line if the cursor is in the middle of the text. With insert off, the carriage return in only added to the file if it is at the end of the file, otherwise the cursor is moved non-destructively to the beginning of the next line.