The function below is part of a script to trim email list from a file into domain list and removes duplicates.

/* define a function that can accept a list of email addresses */
function getUniqueDomains($list) {
    // iterate over list, split addresses and add domain part to another array
    $domains = array();
    foreach ($list as $l) {
        $arr = explode("@", $l);
        $domains[] = trim($arr[1]);
    }

    // remove duplicates and return
    return array_unique($domains);
}

What does $domains[] = trim($arr[1]); mean? Specifically the $arr[1]. What does [1] mean in this context? How come variable $arr becomes an array variable?