What is PHPSESSID?

I'm playing around with cookies. And I dont have any cookies called PHPSESSID.

Do i need it? Can i remove it?

Whats the "function" of it?

if (count($_POST)) {

setcookie("TestCookie", htmlspecialchars($_POST['val']), time()+3600);
}

print_r($_COOKIE);

Prints:

Array
(
    [TestCookie] => blabla
    [PHPSESSID] => el4ukv0kqbvoirg7nkp4dncpk3
)