How to use Product Metafields

extract.liquid

{% assign mf = product.metafields.meta_data %}
{% unless mf == empty %}
  {% for mf in product.metafields.meta_data' %}
    {% capture key %}{{ mf | first }}{% endcapture %}
    {% if key == 'meta_description' %}
      <h1>{{mf | last}}</h1>
    {% endif %}
  {% endfor %}
{% endunless %}