Insert will insert the letter/numebr that you type wherever your cursor happens to be.
I.e., if you type the letters 'ot', place your cursor between the two. If MS Word is in 'insert' mode, and you typ the letter 'u', it will insert the letter.
On the other hand, if you do the same thing, and MS word is in Overwrite mode, the 'u' that you type will over write the 't', leaving you with 'ou'.