what is str_pad in php

<?php 
   $str = "Geeks for geeks"; 
   echo str_pad($str, 26, " Contribute", STR_PAD_RIGHT);  
?> 

<?php 
   $str = "Geeks for geeks"; 
   echo str_pad($str, 21, ":-)", STR_PAD_BOTH);  
?>