what is this ‘:user_id’

The code

PHP:

$params = array( ‘:user_id’ => $_SESSION[‘smth][‘user_id’] );

$sql = "SELECT * FROM `smtble` WHERE `user_id` = :user_id;";

$stmt = parent::query($ssafdsf, $sjrlj);

What does this colon means in front of user_id ?

See original post by ameerulislam43