Объясните что такое fetch_assoc() и как с ним работать

<?php
/* SYSTEM FILE */
$act = isset($_GET['act']) ? htmlspecialchars($_GET['act']) : '';
session_start(); 
ob_start();
 
function fl($m){ 
    if(!is_numeric($m)){
        $m = htmlspecialchars($m);
    } else {
        $m = intval($m);
        $m = abs($m);
    }
    return $m;
} /* FILTER */
 
if(isset($_COOKIE['login'])  && isset($_COOKIE['password'])) {
    $u = $base -> query("SELECT * FROM `users` WHERE `login` = '".fl($_COOKIE['login'])."' && `password` = '".fl($_COOKIE['password'])."' LIMIT 1") -> fetch_assoc();

$result = $db -> query (
  "SELECT `id`, `login`, `email`, `password` FROM `users` WHERE `login` = '$login' AND `password` = '$password'"
) -> fetch_assoc();
 
print_r($result);

class dbClass {
 
  const DATABASE = 'test';
  const HOST = 'localhost';
  const USER = 'root';
  const PASSWORD = '';
 
  private $mysqli;
  private static $instances = [];
 
  public function __construct()
  {
    $this -> mysqli = new mysqli(
      self::HOST, self::USER,
      self::PASSWORD,
      self::DATABASE
    );
  }
 
  private function __clone(){}
  private function __wakeup(){}
 
  public static function instances() : dbClass
  {
    $self = static :: class;
 
    if (!isset(static :: $instances[$self])) {
      static :: $instances[$self] = new static;
    } return static :: $instances[$self];
  }
 
  public function query($sql)
  {
    return $this -> mysqli -> query($sql);
  }
 
}
 
$db = dbClass::instances();