Mysql 数据库有关问题，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'";