What is mean by Differential Daisy Chain UART?
It most likely means that the output (TX) of one UART is fed to the input (RX) on another one. The benefit is that from the host/microcontroller side you need just one UART to communicate with many slave devices in a chain. This is similar to a daisy chain SPI (for example: like chain of shift registers).

This of course has to be supported by the protocol, so for example when you send message to the third device in the chain the other devices must be somehow bypassed.

In this particular case the functionality is explained in "7.3.10 Stacked Daisy-Chain Communications" of the datasheet. Basically your MCU talks via a standard UART to a "master" chip that is connected with differential lines to the other chips in the chain and uses a proprietary protocol.