I'm very confused about server-side documentation.

Actually I have a datatables initilized from DOM, created with a foreach loop in PHP.
The result is 6000 rows... The page freeze for some seconds but at the end of loading all work great: pagination (default set to 50 per page), bFilter, ...

Now i'm trying to migrated to server-side loading but i'm very confusing about documentation.
I tried and my code get results, but pagination is broken (all 6000 results are printed in one page with pagination only 1),... also i don't know meaning of some fields like: "draw" in json ajax response.

How can I fix my code to show only 50 rows on first load of the page?

here is my code:

$('#talbe').DataTable( {
"aaSorting": [[ 0, "asc" ]],
responsive: false,
paging: true,
"pageLength": 50,
"lengthMenu": [ [20, 50, 100, -1], [20, 50, 100, "All"] ],
"info": true,
scrollCollapse: false,
scrollY: false,
scrollX: true,
"columns": [
        { "data": "Field1" },
        { "data": "Field2" },
        { "data": "Field3" },
        { "data": "Field4" },
        { "data": "Field5" },
        { "data": "Field6" }
    ],
"processing": true,
"serverSide": true,
"ajax": "ajax/load.php",
    ...
});

Here my load.php

<?php

$res = get_results_example();

$data = array();

foreach ( $res as $r) {

$temp = array();

$temp['Field1'] = $r['Field1'];
$temp['Field2'] = $r['Field2'];
    $temp['Field3'] = $r['Field3'];
    $temp['Field4'] = $r['Field4'];
    $temp['Field5'] = $r['Field5'];
    $temp['Field6'] = $r['Field6'];

$data[] = $temp;

}

$result = array(
"draw" => $_GET['draw'], //<---------------------------- it's it ok? what it means?
"recordsTotal" => count($res), // in my example is 6000
"recordsFiltered" => count($res), //<--------------------- what value i need to set here?
"data" => $data //<------- should load all 6000 result?
);

echo json_encode($result);

?>