Form validation doubt

Hello,
I have a doubt about a piece of code, where we want to check the the values added by the user are already store in an array or not.

<?php

$validation_error = "";

$username = "";

$users = ["coolBro123" => "password123!", "......"];

 if ($_SERVER["REQUEST_METHOD"] === "POST") {

   $username = $_POST["username"];

   $password  = $_POST["password"];

   if (isset($users[$username]) && $users[$username] === $password){
...

   } else {
...
?>

I thin I understood the first part if isset($users[$username]) where we should be checking if the username already exists.
But what about $users[$username] === $password)?

what are we doing here? I think we are checking the password, but why not like that
isset($users[$password])?
Thanks!

But what about $users[$username] === $password)?

Try calling echo on $users[$username] and $password separately on the line before. You ask a good question because you always want to make sure in conditionals you know what values are being checked.