It changes between insert and overtype modes in most text editing/word processing programs. It's easier if you just play with it yourself to see the difference in action.
Open up something MS Word if you have it, or WordPad should also suffice. Type in something like: The quick brown fox jumped over the lazy dog. Now position the cursor at some random point in the middle of that sentence and start typing something else. The default mode is usually insert, so after typing a little, press the insert key and observe the difference.
<font size="5">Can anyone tell me what the keyboard key labeled "INS" does and when to use this key? Thank you. user470