Allamah Tabatabai states: "Infallibility" means the presence of a quality in the infallible person that protects him or her from doing what is not permissible (e.g. sin or error)". It should be noted that from his viewpoint God's prophets (and all of the imams and specially guided people) possess the absolute infallibility which covers many instances. According to him, "infallibility" is the result of the infallible person's knowledge of the consequences of the sin, that is to say that the infallible person’s knowledge and awareness of the sin's consequences is such that he/she is never conquered and overwhelmed by the rest of the faculties he/she possesses, but has dominance over them.
Concerning "Infallibility", there is another theory that says: "As a result of the infallible’s high levels of knowledge towards God almighty and owing to their affection for God's beauty and perfection, they do not allow themselves to take even a step against His will, and their love and affection for the Deity and the perception of His beauty and glory and their awareness of His endless blessings upon them, brings to their soul such humility and modesty that never allows them to even think of sin let alone committing it".
These two theories both admit that a certain quality develops in the infallible person which constantly drives away disobedience, insubordination and wrongdoing and protects against sin and mistake.
There are two kinds of infallibility; infallibility in knowledge and practical infallibility. The infallibility that the prophets possess comprises both. In other words, both their actions are righteous and according with the truth, and their knowledge is in accordance with reality, originating from a source that accepts no mistake, fault or forgetfulness. That is because one who is infallible has reached a certain level of incorporeal intellect, pure intuition, true vision and spiritual unveiling (In other words, the infallible individual not only knows the truth, he/she sees and witnesses it). In this stage, all mental and spiritual faculties such as one’s imagination and conjectural faculty follow the mind, and satanic temptations here are helpless. The reason behind such a matter is that ignorance, satanic temptations, false illusions and the like, are all a result of one’s imaginative or illusive faculty in which both are connected to the materialistic world, but such isn’t the case with the mind and intellect. In the realm of the mind and intellect, Satan is helpless. Therefore, if one is able to surpass the stages of this materialistic world and somehow overcome the abovementioned faculties and reach the origin of all truths and realities, he/she will have become infallible and as a result of his/her strengthened existence, will be equal to the angels or maybe even higher.
Conclusion: The infallible individual is one who is free of any sin, mistake and forgetfulness throughout his/her life when it comes to comprehending, conveying, and carrying out divine revelation (wahy).
Of course, one should note that infallibility is divided into voluntary and involuntary infallibility. The infallible individual is one who voluntarily doesn’t commit sin. At the same time, in order for it to be absolutely and totally obligatory for us to follow their traditions (actions, sayings, and verifications) without any questioning, and to be able to completely depend on them, an infallibility is granted to them by Allah (s.w.t.) which has nothing to do with their own will and choice and is much higher than the infallibility that they have obtained by themselves. Of course, they are granted it because they deserve it as a result of their voluntary efforts in Allah’s way.
For more information, see Index:
1- The concept of infallibility and the possibility of individuals being infallible,
2- The mother of Imam Zaman (a.s.) and infallibility, question 178 (site: 1395).
 S. Mohammad Husein Tabatabai, Al-Mizan fi Tafsir-al-Quran, v.2, pg.134 (The infallibility of the imams)
 Ibid, v.2, pg. 134-136, v.5, pg. 75-77, Javadi Amoli, Tafsir Mozoo’I Quran Karim, v.3, pg. 197-292.
 Ibid, v.5, pg. 78.
 Jafar Sobhani, Manshoor-e-Javid, v.5, pg.14.
 These points are from Ayatollah Javadi Amoli, Tafsir Mozoo’I Quran Karim, v.3, pg. 199-202.
 See: Seyyed Mohsen Kharrazi, Bidayat-ul-Ma’arif, pg. 254.