explain me a preg_match if clause please

explain me a preg_match if clause please

if(preg_match("/^[\w_.]+$/",stripslashes($_GET['key']))) {
$key = $wpdb->escape(stripslashes($_GET['key']));
}
assuming the key value is = be4e53680e6518cca701ec091258642f0740fe3d
can someone please explain me the if condition ? I`m confused on what
exactly it checks for