Normally, the IDE is in insert mode. This means that any text that is typed will be inserted before text that is present after the cursor.
In overwrite mode, any text that is typed will replace existing text.
When in insert mode, the cursor is a flat blinking line. If the IDE is in overwrite mode, the cursor is a cube with the height of one line. Switching between insert mode and overwrite mode happens with the Insert key or with the Ctrl-V key.