-- Parent referenced table 
pgdb=# create table r (c char primary key );

-- Children referencing table 
pgdb=# create table t (c char primary key );
pgdb=# alter table  t add  FOREIGN KEY (c) REFERENCES r;

-- Due the foreign constraint can  insert on children table(t) only if the value is in the referenced table (r)
pgdb=# insert into r values ('a');
pgdb=# insert into t values ('a');

-- Obviously, cannot insert or delete data not in the parent table
pgdb=#  insert into t values ('b');
ERROR:  insert or update on table "t" violates foreign key constraint "t_c_fkey"
DETAIL:  Key (c)=(b) is not present in table "r".

-- In fact, nor deleting
pgdb=# delete from r;
ERROR:  update or delete on table "r" violates foreign key constraint "t_c_fkey" on table "t"
DETAIL:  Key (c)=(a) is still referenced from table "t".

-- 
pgdb=# ALTER TABLE  r  DISABLE TRIGGER ALL;
ALTER TABLE
pgdb=# delete from r;
DELETE 1

-- don't forget to renable it ...
pgdb=# ALTER TABLE  r  ENABLE TRIGGER ALL ;