An extended public key (xpub) is a master public key created by a corresponding master private key (xpriv). One way to think of an xpub is as a master lock. An xpub generates individual locks to all of the individual keys the xpub generates.
All individual public keys (pubkeys, or "locks" from our analogy) are derived from the xpub. Pubkeys are used to create individual bitcoin addresses. You can build an address with one pubkey (single signature addresses), or you can build an address with three pubkeys (multisignature addresses).
When you upload two xpubs from two hardware wallets to Unchained Capital, this allows for new addresses to be generated for as many multisignature vaults and loans as you would like.
If you want to learn more, one of our engineers Justin Moore has an insightful write-up on xpubs that might be of help.