what is a fieldset html

The <fieldset> is a tool for organizing and grouping related 
items within a form. (Mostly visual for the reader).
 
 <form>
  <fieldset>

    <input type="radio" id="kraken" name="monster">
    <label for="kraken">Kraken</label>

    <input type="radio" id="sasquatch" name="monster">
    <label for="sasquatch">Sasquatch</label>

  </fieldset>
</form>