Doubt sent reply with ajax

Help me understand how this works please. How do you receive in .outer_div the message that is created in edit_banner.php through data by request ajax? I see you have this:

$(".outer_div").html(data).fadeIn('slow');

But I do not understand what is going on in the data. Is it sent in the same ajax or is the response created in edit_banner.php?

This would be the ajax script. Understand that there is a div with class .outer_div where this is received.

<script>
    $("#editar_banner").submit(function(e) {

          $.ajax({
              url: "ajax/editar_banner.php",
              type: "POST",
              data: $("#editar_banner").serialize(),
               beforeSend: function(objeto){
                $("#loader").html("Cargando...");
              },
              success:function(data){
                    $(".outer_div").html(data).fadeIn('slow');
                    $("#loader").html("");
                }
        });
         e.preventDefault();
    });
</script>

on the other hand I have the edit_banner.php

if($_SERVER['REQUEST_METHOD'] == "POST" && isset($_POST["titulo"])){
/* Llamar la Cadena de Conexion*/
include ("../../conexion.php");
// escaping, additionally removing everything that could be (html/javascript-) code
 $titulo = mysqli_real_escape_string($con,(strip_tags($_POST['titulo'], ENT_QUOTES)));
 $descripcion = mysqli_real_escape_string($con,($_POST['descripcion']));
 $descripcion_corta = mysqli_real_escape_string($con,($_POST['descripcion_corta']));
 $url_web = mysqli_real_escape_string($con,($_POST['url_web']));
 $orden = intval($_POST['orden']);
 $estado = intval($_POST['estado']);
 $id_banner=intval($_POST['id_banner']);
 $sql="UPDATE portafolio SET titulo='$titulo', descripcion='$descripcion', orden='$orden', estado='$estado', descripcion_corta='$descripcion_corta', url_web='$url_web' WHERE id='$id_banner'";
 $query = mysqli_query($con,$sql);
// if user has been added successfully
if ($query) {
    $messages[] = "Datos  han sido actualizados satisfactoriamente.";
} else {
    $errors []= "Lo siento algo ha salido mal intenta nuevamente.".mysqli_error($con);
}

if (isset($errors)){

        ?>
        <div class="alert alert-danger" role="alert">
            <button type="button" class="close" data-dismiss="alert">&times;</button>
                <strong>Error!</strong>
                <?php
                    foreach ($errors as $error) {
                            echo $error;
                        }
                    ?>
        </div>
        <?php
    }
    if (isset($messages)){

        ?>
        <div class="alert alert-success" role="alert">
                <button type="button" class="close" data-dismiss="alert">&times;</button>
                <strong>¡Bien hecho!</strong>
                <?php
                    foreach ($messages as $message) {
                            echo $message;
                        }
                    ?>
        </div>
        <?php
    }

}