warning when validating html code

i get this error message

Validation Output: 3 Warnings

Below is a list of the warning message(s) produced when checking your document. Warning Line 27, Column 15: Section lacks heading. Consider using h2-h6 elements to add identifying headings to all sections. <section>

here is my code for that section

</header> <div id="wrapper"> <section> <ul id="gallery"> <li> <a href="img/numbers-01.jpg"> <img src="img/numbers-01.jpg" alt=""> <p>Experimentation with colour and texture.</p> </a> </li> <li> <a href="img/numbers-02.jpg"> <img src="img/numbers-02.jpg" alt=""> <p>Playing with blending modes in Photoshop.</p> </a> </li> <li> <a href="img/numbers-06.jpg"> <img src="img/numbers-06.jpg" alt=""> <p>Trying to create a 80s style of glows.</p> </a> </li> <li> <a href="img/numbers-09.jpg"> <img src="img/numbers-09.jpg" alt=""> <p>Drips created using Photoshop brushes.</p> </a> </li> <li> <a href="img/numbers-12.jpg"> <img src="img/numbers-12.jpg" alt=""> <p>Creating shapes using repitition.</p> </a> </li> </ul> </section> <footer>

can anyone tell me what i have done wrong please