Ijtihaad (اجتهاد’) in Islam means striving to understand the shar’i ruling on the basis of shar’i evidence. It is obligatory for the one who is able to do it. 
Ijtihad is derived from two words: juhd (exertion of effort or energy) and jahd (the forbearance of hardship, namely, striving and self-exertion in any activity that entails a measure of hardship).
Thus, it would be suitable to use jahada for one who carries a heavy load, but not if he/she carries a trivial weight. Ijtihad is the expenditure of effort to arrive at the correct judgment, whether physical (e.g., walking or working) or intellectual (e.g., inferring a ruling or a juristic and linguistic theory) 
Al-Ghazzali (d. 505/1111) defines it as the “total expenditure of effort made by a jurist for the purpose of obtaining the religious rulings.”
Fazlur Rahman (d. 1988) defines it technically as “the effort to understand the meaning of a relevant text or precedent in the past, containing a rule, and to alter that rule by extending, restricting or otherwise modifying it in such a manner that a new situation can be subsumed by a new solution.”
'Amr bin 'Al-'As (May Allah be pleased with him) said:
I heard the Messenger of Allah (ﷺ) saying, "When a judge utilizes his skill of judgement and comes to a right decision, he will have a double reward, but when he uses his judgement and commits a mistake, he will have a single reward." Sahih Al-Bukhari and Sahih Muslim Book 19, Hadith 1856
Ijtihad is one of the most important key words of the Shari`ah, and its goal is defined as comprehending the purpose of the Qur’an and the Sunnah.
The main purpose of law is to maintain the continuity of answering people’s needs. A legal system that answers the questions of its era while offering solutions for future generations can be realized only by constantly renewing itself. In this sense, ijtihad has very important functions in keeping the law alive, which is one of the Shari`ah’s main dynamics.
One who makes a Ijtihad is called a Mujtahid.
The Mujtahid must have knowledge of the shar’i evidence and knowledge of the basic principles (usool) and scholarly views which, if he knows them, he will be able to derive rulings based on that evidence without unwittingly going against scholarly consensus. If these conditions are met in his case, then he may engage in ijtihaad.
Ijtihaad may be focused on a narrow area, so a person may research one issue of knowledge and examine it thoroughly, and become a mujtahid with regard to that issue, or he could focus on one aspect of knowledge, such as issues having to do with tahaarah (purification), which he researches and examines, and thus becomes a mujtahid in that area. 
By the beginning of the fourth century of the hijra (about AC 900), the point had been reached when the scholars of all schools felt that all essential questions had been thoroughly discussed and finally settled, and a consensus gradually established itself to the effect that from that time onwards no one might be deemed to have the necessary qualifications for independent reasoning in law, and that all future activity would have to be confined to the explanation, application and, at the most, interpretation of the doctrine as it had been laid down once and for all.
This 'closing of the door of ijtihad', as it was called, amounted to the demand for Taqlid, a term which had originally denoted the kind of reference to companions of the Prophet ﷺthat had been customary in the ancient schools of law, and which now came to mean the unquestioning acceptance of the doctrines of established schools and authorities.
However this concept is truly in error and the door was never closed. Islamic legal theory ijtihad was reckoned indispensable in legal matters because it was the only means by which Muslims could determine to what degree their acts were acceptable to Allah.
The idea of closing the gate of ijtihad or the notion of the extinction of mujtahids did not appear during the first five Islamic centuries. This is entirely in consonance with the fact that the practical and theoretical importance of ijtihad had not declined throughout this period. Hence this is an incorrect claim. 
 Was the gate of Ijtihad Closed, Waeal B Haqq, Cambridge University Press , http://www.jstor.org/stable/162939