Cold storage wallets are the opposite of hot wallets. A hot wallet is connected to the internet and is therefore vulnerable to hacks. With a cold storage wallet, all your private keys are stored offline so it is secure from hacks.
There are multiple ways to build a cold storage wallet, but one of the easiest is to have the private keys generated on paper, which is then stored in an offline locker or a safe. In that way , your keys are completely separated from the internet and are safe from all types of hacks and the only way to access them is if you can physically go to the locker or safe where the keys are stored.