My original answer is below. I took a second look at the device and I had to update my answer because my initial answer was about a similar device for mains power, but this one apparently is for 24VDC. The idea remains the same though:
You connect two entirely independent DC power supplies to the input of the device and the load on its output. When one of the two power supplies fails, the other one automatically takes over the load. Ultimately the DC power supplies are fed from independent mains phases so you survive an interruption of mains power and a failing DC power supply.
The device is connected to two independent phases of the mains power supply. When one phase fails, the device switches over to the other phase. That way the power supply remains uninterrupted in case of a first fault condition (eg. a blown fuse).