Cold Storage is a crypto account that is never exposed to the broader world via the internet. Think safety deposit box in a bank that is only accessed when it's absolutely necessary and only by specific individuals. It is therefore the most secure account type and the bulk of a companies crypto assets should ideally sit in Cold Storage. An example would be a hardware wallet stored in a safety deposit box, the private key for the wallet never leaves the hardware device.
Within a Rehive context Cold Storage is usually interacted with to either move a small percentage to the Hot Wallet or the Warm Storage is "sweeped" (funds moved off) to the Cold Storage account.