It is called "Overtype". When Overtype Mode (OVR) is activated, text that is
entered replaces text to the right of the cursor, one character at a time.
When Overtype Mode is deactivated, Insert Mode (INS) is activated.
When Insert Mode (INS) is activated, text that is entered is inserted to the
left of the cursor, one character at a time.
It is common mistake (even by me) to somehow "accidentally" hit the Insert
key on the keyboard.
In any program, all you need to do is to hit the Insert key on the keyboard
to disable Overtype Mode and (at the same time) enable Insert Mode.
In some programs (i.e. Microsoft Word) have an indicator in the status bar
which indicates whether or not Overtype Mode is on. If you look at the
status bar to the right of the 2 large white (or grey) boxes, you'll see
some smaller boxes. One of them has the letters "OVR" in it, which is short
for "Overtype". If "OVR" is grayed out, then Overtype Mode is disabled. If
"OVR" is shown in black text, then Overtype Mode is enabled. To disable it,
position the mouse over the box with the letters "OVR" in it and
double-click it.
