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 address generation.
you to view all your wallet activity without also having access to transfer the assets in those addresses. It's like having a Read-Only version of all your wallet transactions and balances from the very first time you started using your wallet onwards, but without the ability to transfer those balances.
There is an individual XPUB for each of your wallet assets (e.g., one for Bitcoin, one for Ethereum).
XPUBs come in handy when you're using 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.