Hello and welcome to Bleepingcomputer.
The INS key is short for INSert.
It is primarily used to switch between the two text-entering modes on a personal computer(PC) or word processor. The first is overtype mode, in which the cursor, when typing, overwrites any text that is present on and after its current location. The other is insert mode, where the cursor inserts a character at its current position, forcing all characters past it one position further. The insert/overtype mode toggling is not global for the computer or even for a single application but rather local to the text input window in which the Insert key was pressed.
Please check this wiki answer for a more detailed explanation. http://en.wikipedia.org/wiki/Insert_key
I hope this has answered your question.
Edited by MrBruce1959, 27 May 2011 - 05:16 AM.