INS indicates Insert Mode: typing a character inserts it at caret position.

OVR indicates Overwrite Mode: typing a character replaces the character found at caret position by the typed character. If the caret is positioned at the very end of a text (or comment or processing-instruction) node, then typed characters are simply inserted there.

Pressing the INS/OVR button allows to switch from Insert Mode to Overwrite Mode and vice versa. The shortcut associated to this button is by default Esc I on the Mac and Esc Insert (press Esc, release Esc, then press Insert) on the other platforms.