It is a relay with time overcurrent (ANSI/IEEE Device 51) and instantaneous overcurrent (ANSI/IEEE Device 50)functions connected to the neutral (or residual) of wye-connected CTs.
A 50/51N relay can be set more sensitively than the same relay connected directly to phase current because it only senses zero sequence currents (i.e., unbalanced phase to neutral loads or ground faults).