what we mean by the Rules of Natural Justice and what are these rules?

Rules of Natural Justice are the rules and procedure required to be followed by any person who adjudicates upon disputes between or rights of others. There simply rules and procedure required to be followed by the person determining the rights of the other. The rules of natural justice are: 

1). Rules against bias; 

2). No man is judge of his own cause; (nemo judex in causa sua) and 

3). The right to be heard/hear the other side (audi alteram partem)

What happens on a decision given on violation of the rules of natural justice?

The decision given in violation of the rules of natural justice is void, so, the aggrieved has a right to challenge the same by way of judicial ‘review’