what is digital relay?

Answer 1

A flip flop.

Answer 2

It might be considered a flip-flop.

More likely it is a relay that has a logic-level input (i.e. 5 Volt) to energize the coil; or if it is a solid-state relay, a logic level input to turn on the LED as part of the optoisolation circuit, to turn on the output swicthing semiconductor (i.e. Triac, SCR, power CMOS transmission gate).

In either case, it is a device that interfaces logic level (digital) to higher voltage, higher power circuitry, while isolating the logic.

.