What's the "for" for in a label tag?

Just ran across a for parameter in an HTML label tag:

<label for="required-firstname"> First Name </label>
<small>*</small>
<input name="required-firstname" type="text" tabindex="2" 
       id="required-firstname" size="24" maxlength="40">

I'm converting this form to a PHP processed script, can I get rid of the for= parameters? (And out of curiosity, what does it do?)