Mysql 数据库问题，该语句是什么意思

$query = "select orderid from orders where                 customerid = $customerid and                 amount > ".$_SESSION['total_price']."-.001 and //这句是什么意思？                amount < ".$_SESSION['total_price']."+.001 and  //这句是什么意思？                date = '$date' and                order_status = 'PARTIAL' and                ship_name = '$ship_name' and                ship_address = '$ship_address' and                ship_city = '$ship_city' and                ship_state = '$ship_state' and                ship_zip = '$ship_zip' and                ship_country = '$ship_country'";