A 'cold storage' or 'cold wallet' is a way to secure cryptocurrencies. It involves storing cryptocurrencies offline, away from internet access. Keeping cryptocurrencies offline substantially reduces the threat from hackers.
The method of cold storage is less convenient than encrypting or taking a back up. It is usually done by keeping some money in the system for regular spending and putting the rest in a cold storage device. This reduces the effort of digging out coins from the cold storage every now and then for everyday use.