Its meaning is: the Knot (Al-'Uqda) of which al-Sadiq spoke is that which the Messenger of Allah tied on the Day of Ghadir concerning the Wilaya of the 'Ayn [Ali].

Another answer: The Knot was established by Abu Shu'ayb [Muhammad bin Nusayr], and likewise tightened and magnified by al-Husayn bin Hamdan [al-Khasibi]. Indeed, whoever deviates from the path of al-Husayn is among those who have untied the Knot.

Another answer concerning the question of "Fulfilling Contracts" and exiting the state of Ihram [ritual consecration]:

The saying of the Almighty: {O you who believe, fulfill the contracts (al-'uqud). Lawful for you are the beasts of cattle (bahimat al-an'am)} (Al-Ma'idah: 1). As for the "contracts," the exiting from Ihram, and the "lifting of shackles and burdens" from the believers—this refers to the Knowledge of the Inwards (Ma'rifat al-Bawatin). There is nothing Outward except that it has an Inward; we deduce the Inward through its Outward. Likewise, the Science of Tawhid (Divine Unity), in which lies salvation, appeared until it seemed nearly no one was ignorant of it, yet it remained hidden until it seemed nearly no one knew it.

Our Master al-Sadiq said:

"Whoever claims to know Allah by the Outward (Zahir) has ignored more than he has learned.