As described in this article, everything starts with the seed phrase which is generated in accordance with one of BIP44 standards. All the words for any seed phrase are taken from a 2,048 word set and arranged in a unique order.
Passphrase is an optional BIP39 part of the seed phrase. It may contain any words and digits of your choice. Passphrase is never backed up on your device or elsewhere. It is highly desirable to write it down or memorize.
When restoring a wallet with a seed and passphrase, you can skip the passphrase step in OWNR. This is done to make OWNR wallet more intuitive and save your time, but we’re not sure you’ll find the same in other HD wallets, so be careful.