sql - Simple PHP data_base functionality [explanation needed]

Can someone explain to me this DATABASE.PHP (Each Function )code i did not quite understand each function this my first project and i' quite struggling with this i'm creating a job Listing project

what's the purpose of function __construct() || query() || bind() ||execute()|| resultset() || single()

<?php
class Database{
private $host = DB_HOST;
private $user = DB_USER;
private $pass = DB_PASS;
private $dbname = DB_NAME;

private $dbh;
private $error;
private $stmt;

public function __construct(){
    // Set DSN
    $dsn = 'mysql:host='. $this->host .';dbname='. $this->dbname;

    // Set Options
    $options = array(
            PDO::ATTR_PERSISTENT => true,
            PDO::ATTR_ERRMODE => PDO::ERRMODE_EXCEPTION
    );

    // PDO Instance
    try{
        $this->dbh = new PDO($dsn, $this->user, $this->pass, $options);
    } catch(PDOException $e) {
            $this->error = $e->getMessage();
    }
}

public function query($query){
        $this->stmt = $this->dbh->prepare($query);
}

public function bind($param, $value, $type = null){
        if(is_null($type)){
                switch(true){
                    case is_int ( $value ) :
                        $type = PDO::PARAM_INT;
                        break;
                    case is_bool ( $value ) :
                        $type = PDO::PARAM_BOOL;
                        break;
                    case is_null ( $value ) :
                        $type = PDO::PARAM_NULL;
                        break;
                    default :
                        $type = PDO::PARAM_STR;
                }
        }
        $this->stmt->bindValue($param, $value, $type);
}

public function execute(){
    return $this->stmt->execute();
}

public function resultSet(){
    $this->execute();
    return $this->stmt->fetchAll(PDO::FETCH_OBJ);
}

public function single(){
    $this->execute();
    return $this->stmt->fetch(PDO::FETCH_OBJ);
}
   }
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