isset and _get explain to me how this works?

<?php if (isset($_GET["status"]) AND $_GET["status"] == "thanks") { ?>
                <p>Thanks for the email! I&rsquo;ll be in touch shortly!</p>
            <?php } else { ?>

                <p>I&rsquo;d love to hear from you! Complete the form to send me an email.</p>

                <form method="post" action="contact.php">

                    <table>
                        <tr>
                            <th>
                                <label for="name">Name</label>
                            </th>
                            <td>
                                <input type="text" name="name" id="name">
                            </td>
                        </tr>
                        <tr>
                            <th>
                                <label for="email">Email</label>
                            </th>
                            <td>
                                <input type="text" name="email" id="email">
                            </td>
                        </tr>
                        <tr>
                            <th>
                                <label for="message">Message</label>
                            </th>
                            <td>
                                <textarea name="message" id="message"></textarea>
                            </td>
                        </tr>                    
                    </table>
                    <input type="submit" value="Send">

                </form>

            <?php } ?>