user authentication

[DeMuro1]

ok so I'm still pretty fresh to php and MySQL, but I want to do a few things and I'm a little uncertain of a few things so firstly

In this little piece of script, when the database is queired, does username and password need to match for a value other than 0 to be stored in $num....

Also I see the variable $valid all over the plave in tutorials. I've seen it equal tons of things among which are.....good, ok, bad, valid, no.

So my second question-isthe variable $valid a special variable or do people just use it and assign whatever values they want for easy use later on....what would be the benifit of this?

Thanks

PHP Code:

$sql = "SELECT * FROM users WHERE username='$user' and password='$pass'";
                $result = mysql_query($sql);
                $num = mysql_numrows($result);
                if($num != "0") {
                        $valid = 'yes'; 


Also...does anyone know what

PHP Code:

 echo "<META HTTP-EQUIV=\"Refresh\" CONTENT=\"1; URL=index.php\">"; 


That is