Without aria-labelledby:
<label for="alb1">Name</label>
<input id="alb1" type="text">
<p>Please give your first and last name</p>
With aria-labelledby:
<label for="alb1" aria-labelledby="alb2-description">Name</label>
<input id="alb1" type="text">
<p id="alb2-description">Please give your first and last name</p>