The “second negative” is also called the “double negative.” Don’t confuse it with the “negative double” – they are totally different bids.
Partner opens 2♣
When partner opens 2♣ and you have a crummy hand, you first respond 2♦. This is just a negative bid, showing 0 to 7 points. But if you are truly dreadful, you need to turn partner off from his lofty dreams by giving him a “second negative.”