The status area below the menubar indicates whether the Editor is in insert or replace mode. 

In insert mode, the cursor is a thin vertical line positioned between any two adjacent characters in a row marking the insertion point. New characters appear at the insertion point, and any text to the right of the cursor shifts right.
 
In replace mode, the cursor is a solid block one character width in size, positioned directly over top of an existing character position. New characters replace existing ones, and the cursor moves to mark the next character position. No text is shifted.
 