I was hoping someone could explain to me what the ? -1 : 1; mean in the below ternary operator? Many thanks

```
<?php
 $people = array(
 array( "name" => "hank", "age" => 39 ),
 array( "name" => "Sarah", "age" => 36 ),
 );
 usort( $people, function( $personA, $personB ) {
 return ( $personA["age"] < $personB["age"] ) ? -1 : 1;
 } );

 print_r( $people );

```