Tells you whether you are in Paste or Reprint mode.
From wikipedia :
It is mainly used to switch between two text input modes on a personal computer (PC) or word processor. The first is an overprint of mode, in which the cursor overwrites any text that is present after its current location when typing. The other is the insert mode, where the cursor inserts a character at its current position, forcing all characters to go past it one more position. The insert / overprint mode switching is not global for the computer or even for one but rather locally for the text input window in which the Insert key was pressed.
In early textual computing environments and terminals, when the cursor was in overtype mode, it was presented as a block that surrounded the entire letter to be surpassed; when in insert mode, the cursor consisted of a vertical strip, which is very common among modern applications or a flashing underline at the position where a new character will be inserted.