Ummmmm......would you believe TWO
the backslash (\) is used to "escape" something--most often to change meaning from special to ordinary, but it can be the other way too.
In this case, there is no special character and nothing to escape, so "\" does nothing. In you example, I'm betting that it simply replaced every "x" with "x"....
Some examples (in regexes such as used in the sed "s" command):
. any character
\. literal period
$a value of variable "a"
\$a literal "\$a"
\ often means nothing
\x same as "x"
\\ literal "\"
\\\. literal "\", followed by a literal "\." In other words "\\\." means literal "\."
If you're not confused yet, we can try harder....
Finally, don't use sed -i until you are sure your code is working. It changes the file in place. You are always safer doing:
sed '<<stuff>>" oldfile > newfile