What do you mean by load rejection?

In real world load rejection is when there is a

-Fault on the transmission line which causes circuit breaker to trip and as a result of this tip faulty line has been dropped.This event would be seen by the system, connected to other end of line, as load loss and event is called load rejection.

in layman terms

A load rejection is term used when load has rejected the supply or to remember easily

it's when load (receiving end) decided not to talk fully to supply (sending end).

Take a very basic example suppose you are running a motor which is drawing current

now for one or another reason there is fault in the line feeding current to motor and this line gets disconnected. Now sending end is still sending the current but receiving end (motor) can't receive it, this is called load rejection.