"the thing has been decided" The principle that a final judgement of a competent court is conclusive upon the parties in any subsequent litigation involving the same cause of action.
The Doctrine of Res-judicata is based on the following three maxims:

1. Nemo Debet Lis Vexari Pro Una Et Eaden Causa – No man should be vexed twice over for the same cause;
2. Interest Republicate Ut Sit Finis Litium- It is in the interest of the State that there should be an end to a litigation; and
3. Res Judicata Pro Veritate Occipitur – A judicial decision must be accepted as correct.
The doctrine of Res Judicata has been explained in the simplest possible manner by Das Gupta, J., in the case of Satyadhyan vs. Smt. Deorajin Debi in the following words:

The principle of Res Judicata is based on the need of giving finality to judicial decisions. What it says is that once a res is judicata, it shall not be adjudged again.
Primarily it applies as between past litigation and future litigation. When a matter whether on a question of fact or a question of law has been decided between two parties in one suit or proceeding and the decision is final, (neither because no appeal was taken to a higher Court or because the appeal was dismissed, or no appeal lies), either party will not be allowed in a future suit or proceeding between the same parties to canvass the matter again.
Section 11 reads thus:

No Court shall try any suit or issue in which the matter directly and substantially in issue has been directly and substantially in issue in a former suit between the same parties, or between parties under whom they or any of them claim, litigating under the same title, in a Court competent to try such subsequent suit or the suit in which such issue has been subsequently raised, and has been heard and finally decided by such Court.
Explanation I:

The expression “former suit” shall denote a suit which has been decided prior to the suit in question whether or not it was instituted prior thereto.
Explanation II:

For the purposes of this section, the competence of Court shall be determined irrespective of any provisions as to a right of appeal from the decision of such Court.
Explanation III:

The matter above referred to must in the former suit have been alleged by one party and either denied or admitted expressly or impliedly, by the other.
Explanation IV:

Any matter which might and any ought to have been made ground of defence or attack in such former suit shall be deemed to have been a matter directly and substantially in issue in such suit.
Explanation V:

Any relief claimed in the plaint, which is not expressly granted by the decree, shall, for the purposes of this section, be deemed to have been refused.
Explanation VI:

Where persons litigate bonafide in respect of a public right or of a private right claimed in common for themselves and others, all persons interested in such right shall, for the purposes of this section, be deemed to claim under the persons so litigating.
Explanation VII:

The provisions of this section shall apply to a proceeding for the execution of a decree and references in this section to any suit, issue or former suit shall be constructed as references, respectively, to a proceeding for the execution of the decree, question arising in such proceeding and a former proceeding for the execution of that decree.
Explanation VIII:

An issue heard and finally decided by a Court of limited jurisdiction, competent to decide such issue, shall operate as Res Judicata in a subsequent suit, notwithstanding that such Court of limited jurisdiction was not competent to try such subsequent suit in which such issue has been subsequently raised.