For now don't worry about JSON. The following converts JSON into a regular PHP array. An array which has more arrays inside it ( Multidimensional array)
//convert json object to php associative array
$data = json_decode($jsondata, true);
Simple explanation: For an array
$carsArr = array(
array("Volvo", 22, 18),
array("BMW", 15, 13),
array("Saab", 5, 2),
array("Land Rover", 17, 15)
);
$carsArr[0] - first item in the array . Basically $carsArr[0] is equal to ("Volvo", 22, 18)
Now $carsArr[0][1] would be the second item of $carsArr[0] . That is '22