Most text editors and word processors have two text entry modes from which you can choose. In insert mode, the editor inserts all characters you type at the cursor position (or to the right of the insertion point). With each new insertion, the editor pushes over characters to the right of the cursor or pointer to make room for the new character.
If insert mode is turned off, the editor overwrites existing characters instead of inserting the new ones before the old ones. This is often called overstrike (or overwrite) mode. Most PC keyboards have an Ins key that lets you switch back and forth between insert and overwrite modes.