Marja'eyah in the sense of giving a fatwa is a jurisprudential term the opposite of which is the term "Taqlid". That is to say, whoever is a Marj'a, others are his muqallid (followers). In order to analyze the concept of Marja'eyah, we must explain the meaning of "Taqlid".
Taqlid in Persian language has a load of meaning which implies following someone without knowing the reason. It is a blind imitation which is reproached. Allama Iqbal says in his famous poem as such:
Khalq ra taqlid shan bar baad daad
Ay doosad la'nat bar ein taqlid baad.
[The people were destroyed owing to their blind imitation. May a thousand curses be upon such an imitation.]
Iqbal's poem refers to blind imitation. However, in Islamic jurisprudence the term "taqlid" means referring to an expert in a technical matter. Hence, unlike the first meaning which is unwanted and rejected, the second meaning of "taqlid" is completely acceptable and rational. The most important reason for the permissibility of taqlid in religious affairs rests in the very rational point that every layman or inexpert individual turns to an expert for advice on technical and specialized issues. All the textual proofs and evidence of taqlid including the Quranic verse "Ask those who know, if you do not know" and also narrations imply the same meaning of taqlid which is acceptable to every wise individual.
Having said that, Marja is a jurist competent enough to deduce precise inferences regarding the commandments from the religious sources [i.e. the holy Qur'an and the Sunnah of the holy Prophet].
1. Mahdi Hadavi Tehrani, Wilayah and Religiousity, Khana-e Khirad Cultural Institute, Qom, second edition, 1380.
 - Nahl: 43 "فاسئلوا أهل الذکر إن کنتم لا تعلمون"