你是指定语从句中的关系代词吧?
关系代词在定语从句中,起到连接主句和从句的作用,同时,它在从句中也充当一定的成分.
the man is a teacher.she married him.
可以改成：The man whom she married is a teacher.
在这个句子里,关系代词就是whom,定语从句是：whom she married,其中whom在从句中充当宾语,相当于him,而它又连接了主句和定语从句.
当然还可以改成：
She married the man who is a teacher.
这个句子里,主句是she married the man,定语从句是who is a teacher,who作为关系代词,既充当了从句中的主语,也把主句和从句连成一个句子.