that bit of code for your htaccess file , really has nothing to do with wordpress in particular.
a 404 error means that someone attempted to view a page on your website that does not exist.
If the webpage used to exist and you want visitors to go to some other webpage/the new version of that old webpage, etc.
then, you use the redirect in your htaccess file.
There is probably already a htaccess file in your main directory.
if so, open it in a plain text editor and add that line to the bottom of it.
of course you have to replace his examples with the path/file name of the old file and the path to the file you want people to be redirected to.
for instance.
let's say your file that produces the 404 error was located here:
oldfolder/oldfile.html
and you want to redirect anyone attempting to access that file to;
newfolder/newfile.html
then, your htaccess file would have this in it:
Redirect 301 /oldfolder/oldfile.html
http://yourdomain.com/newfolder/newfile.html
it must all be on one line and the spaces are important.
robert