The term, "Marja" refers to a highly skilled and experienced scholar of Islam who has reached a superior level of ijtihad (the ability to derive the laws of the shari‘ah through a deep analysis of its sources) and alamiyyah (achieving a high level of scholarship in relation to all other mujtahīdīn).
To understand the concept in the most simplest way, consulting a Marja is akin to consulting a very high-level medical specialist whose expertise is beyond that of our family doctor’s knowledge.
Thus, the marja may be seen as a very high-levelled and skilled specialist in Islamic law, jurisprudence, ethics and philosophy who helps individuals to not only deal with often unique, unprecedented and often unusual religious issues, but also helps to guide the Islamic ummah by delivering verdicts based on contemporary societal issues which may come up in the life of believers.
A thorough and comprehensive analysis of the concept of Marjaiyyah (including a discussion of the various quranic ayah and ahadith that have provided a basis for the development of this notion) has been recently published by the World Federation in addition to a 4 part series of video lectures on the topic of Marjaiyyah.
4 part video series: