Think of a unified address like a universal travel adapter that lets you plug into different sockets. It’s a single address that works across all Zcash pools, transparent and shielded. If your wallet supports the autoshieldeing feature, too, any ZEC you receive to this unified address can be shielded (or made private) by default. When you send ZEC from a unified address, your address is shielded, regardless of the receiver’s address type.