suppose two table EMPLOYEE(eid,ename) and DEPARTMENT(Did,Dname) and now we make eid as a foreign key , now if we delete any Eid from EMPLOYEE and that Eid was refrenced by foriegn key of DEPARTMENT table then to maintain refrential integrity we hav to also delete that coressponding refrenced key from DEPARTMENT.
but if we donot want to loss data of DEPARTMENT table so instead of deleting that refrenced key of foriegn key we put null values , by this we do not loss the data of DEPARTMENT table