why does word sometimes remove letters as I'm typing

[Marshams]

Every so often something changes when I go back to insert text in the middle
of a sentence. For every letter I type the text that did follow is 'eaten up'

 

[Tom Willett]

You have apparently hit the "insert" key. Punch it again to undo.

: Every so often something changes when I go back to insert text in the
middle
: of a sentence. For every letter I type the text that did follow is 'eaten
up'

 

[Marshams]

That's it. Thanks