As you have seen, generating HD keys is easy. However, what if we want an easy way to transmit such a key by telephone or hand writing?
Cold wallets like Trezor, generate the HD Keys from a sentence that can easily be written down. They call such a sentence “the seed” or “mnemonic”. And it can eventually be protected by a password or a PIN.
The language that you use to generate your 'easy to write' sentence is called a Wordlist.