The solemn promise made by a Freemason on his admission into any Degree is technically called his obligation. In a legal sense, obligation is synonymous with duty. Its derivation shows its true meaning, for the Latin word obligatio literally signifies a tying or binding. The obligation is that which binds a man to do some act, the doing of which thus becomes his duty. By his obligation, a Freemason is bound or tied to his Order. Hence the Romans called the military oath which was taken by the soldier his obligation, and, too, it is said that it is the obligation that makes the Freemason.
Before that ceremony, there is no tie that binds the candidate to the Order so as to make him a part of it; after the ceremony, the tie has been completed, and the candidate becomes at once a Freemason, entitled to all the rights and privileges and subject to all the duties and responsibilities that enure in that character. The jurists have divided obligations into imperfect and perfect, or natural and civil. In Freemasonry there is no such distinction.
The Masonic obligation is that moral one which, although it cannot be enforced by the courts of lav, is binding on the party who makes it, in conscience and according to moral justice. It varies in each Degree, but in each is perfect. Its various clauses, in which different duties are prescribed, are called its points, which are either affirmative or negative, a division like that of the precepts of the Jewish law. The affirmative points are those which require certain acts to be performed; the negative points are those which forbid certain other acts to be done. The whole of them is preceded by a general point of secrecy, common to all the Degrees, and this point is called the tie.
the source: Mackey’s Encyclopedia of Freemasonry