Fieldset and Legend Elements

<fieldset> <legend> … </fieldset>

<form action="/preferences" method="post" name="preferencesForm">
  <fieldset>
    <legend>Contact Preferences</legend>
    <label>
      <input type="checkbox" name="contact" value="email" />
      Email
    </label>
    <label>
      <input type="checkbox" name="contact" value="phone" />
      Phone Call
    </label>
  </fieldset>

  <input type="submit" value="Save Preferences" />
</form>

fieldset

The fieldset element provides a grouping mechanism for a set of inputs as well as their labels.

Aside from providing a container for CSS styling, fieldsets allow disabling groups of inputs at once.

legend

The legend element provides a caption for its parent fieldset.

MDN legend and fieldset documentation