Clause 1. No State shall enter into any Treaty, Alliance, or Confederation; grant Letters of Marque and Reprisal; coin Money; emit Bills of Credit; make any Thing but gold and silver Coin a Tender in Payment of Debts; pass any Bill of Attainder, ex post facto Law, or Law impairing the Obligation of Contracts, or grant any Title of Nobility.
Under the Constitution, only the federal government (not the various state governments) has the power to conduct foreign diplomacy or print money. And the states are barred from doing many of the same things that the federal government also can't do: they can't pass bills of attainder or ex post facto laws, they can't pass laws that break contracts, and they can't grant state-level titles of nobility. This clause is included mainly to ensure that the states don't start acting like independent countries, undermining the national government.
Clause 2. No State shall, without the Consent of the Congress, lay any Imposts or Duties on Imports or Exports, except what may be absolutely necessary for executing its inspection Laws: and the net Produce of all Duties and Imposts, laid by any State on Imports or Exports, shall be for the Use of the Treasury of the United States; and all such Laws shall be subject to the Revision and Control of the Congress.
Again, the states aren't independent nations, so they can't charge tariffs on imports from other states.
Clause 3. No State shall, without the Consent of Congress, lay any Duty of Tonnage, keep Troops, or Ships of War in time of Peace, enter into any Agreement or Compact with another State, or with a foreign Power, or engage in War, unless actually invaded, or in such imminent Danger as will not admit of delay.
And last but not least, the states aren't allowed to run their own armies or start their own wars.