Problem editing text bar in IM window

I'm doing something bad and I dont know what. Somehow my ability to insert text into arbitrary positions in the text bar of my IM window is compromised. A heavier vertical cursor bar appears and I cant insert text. I have to retype everything after the insertion position.
What is it that creates this problem and how do i get it to revert to 'normal' insertion practice.

Thanks very much for any suggestions or help.

it means you've probably hit the insert key (default mode IS insert) and changed to overwrite mode.