It prevents the browser from doing MIME-type sniffing. Most browsers are now respecting this header, including Chrome/Chromium, Edge, IE >= 8.0, Firefox >= 50 and Opera >= 13. See :
Sending the new X-Content-Type-Options response header with the value
nosniff will prevent Internet Explorer from MIME-sniffing a response
away from the declared content-type.
EDIT:
Oh and, that’s an HTTP header, not a HTML meta tag option.
See also : http://msdn.microsoft.com/en-us/library/ie/gg622941(v=vs.85).aspx