How to use Redirect to my WordPress

I made a new website with WordPress and want to upload it. I created a subdomain of my main site (which is in Joomla).
I would just like to redirect the main site by modifying the index.php page, I find it too risky to start modifying the database and I am afraid I am wrong.
By browsing the WEB I found the following code
<? php
// 301 Moved Permanently
header (“Location: https://www.example.com/another-page.php&#8221;, true, 301);
exit ();
?>

When do you think and what does the code for the 301 mean, there are sites that only offer
<? php
header (“Location: https://www.example.com/another-page.php&#8221;);
exit ();
?>

Thank you