Laymen's terms:
A recursive function is a function that calls itself.
 You can set up a condition, known as a base case to make the function stop as it keeps calling itself. HEre is an example factorial :
function fact($n) {
  if ($n === 0) { // our base case
     return 1;
  }
  else {
     return $n * fact($n-1); // <--calling itself.
  }
}
I hope it helps you.