After counting the omer each night, there is a short prayer HaRachaman Hu Yachazir Lanu Avodat Beit HaMikdash Limkomo Bimheira Be'yameinu. Amein Selah Amen is the usual ending of a prayer. Why is the additional word selah necessary? What is the meaning of "Amein Selah"? What do the two words together mean versus each word separately? Thanks.