What is the purpose of the <fieldset> and <legend> elements?

<fieldset> groups related form controls, and <legend> gives that group a caption.

<fieldset>
  <legend>Personal Information</legend>
  <input type="text" name="name">
  <input type="email" name="email">
</fieldset>

The browser draws a border around the group with the legend set into it. Screen readers use the legend as the group's label, which matters most for radio buttons that share one question.

The disabled attribute on a fieldset disables the controls inside it at once, and the form does not submit them. Controls inside the <legend> itself stay enabled.