Newbie question - blank before the cursor

[Klemens Barfus]

hello together,
I am a newbie to emacs and have a question that will be very simple to 
answer:
When I use emacs on the QVT-Terminal under Windows, I often get in a 
mode, where there is a blank before the cursor and when I move the 
cursor sign, the blank moves too and the characters before the blank are 
deleted. I can not see all written characters in this mode, so that my 
generated shell scripts often do not work.
What kind of mode is it ?
How do I get in this mode ?
And how do I leave the mode ?

Thanks for your help in advance !

Klemens