What does the => operator mean in the following code?

What does the => operator mean in the following code?

$foo = array('car', 'truck', 'van', 'bike', 'rickshaw');

foreach ($foo as $i => $type) {

    echo "{$i}: {$type}\n";

}

//Output:    0: car 1: truck 2: van 3: bike 4: rickshaw