Jaxx Liberty follows the BIP32 standard in generating wallet addresses. XPUB is short for Extended Public Key and is part of this industry standard for wallet addresses generation.
XPUBs allow you to have a look at all your wallet activity without also having access to transfer the assets in those addresses. It's like having only a Read Only version of all your wallet transactions and balances from very first time you started using your wallet and onwards, but without the ability to also transfer those balances.
There is an individual XPUB for each of your wallet assets (e.g. one for Bitcoin, one for Ethereum etc.).
XPUBs come in handy when you're using a tax software for example, and that software needs access to all your wallet transactions to calculate your taxes. By sharing your XPUB with that software, you're safely giving them access to the transaction information without also giving them full control access to the digital assets held in that wallet.
We are working on adding the function to access your XPUB directly in the Jaxx Liberty interface. This is not currently available in Jaxx Liberty, but it will be soon.