Union Government
Subject to this Constitution, the Supreme Court has exclusive original jurisdiction over disputes between the Government of India and one or more States, between the Union and some States on one side and other States on the other, or between two or more States, where the dispute involves a legal right - subject to an exception for certain pre-Constitution treaties and agreements.
Only the Supreme Court - no other court - can hear genuine legal disputes between the central government and state governments, or between two or more states themselves. This is called its "original jurisdiction" because such cases start directly in the Supreme Court rather than working their way up through lower courts.
If two states have a serious legal dispute over sharing river water or a boundary, and it involves an actual legal right rather than just policy disagreement, that dispute would be filed directly in the Supreme Court under this article - not in any state's own courts.