A hardware wallet is a device that resembles a flash drive in size, equipped with a small display (in rare cases, without a display) and navigation buttons. The device provides cold storage of private keys in a strictly isolated environment. Let me remind you that a private key is an alphanumeric string that allows anyone who gets access to it to spend cryptocurrency.
Hardware wallets belong to the cold method of storing cryptocurrencies, since these keys are not located in a general-purpose operating system such as Windows, Mac, Linux, where a huge variety of applications, processes and programs run, but on a strictly designed microchip for this purpose. Private keys never leave the device; instead, a signed transaction enters the OS, which does not disclose any useful information to a malicious program or attacker.