Full Faith and Credit Clause is a clause from Section 1 of Article IV of the US Constitution that describes interstate relations.
Section 1 of Article IV says: “Full Faith and Credit shall be given in each State to the public Acts, Records, and judicial Proceedings of every other State. And the Congress may by general Laws prescribe the Manner in which such Acts, Records, and Proceedings shall be proved, and the Effect thereof.”
Thus, Section 1 points out that all states shall respect the laws of other states. Responsibility for implementing this clause lies with Congress, which may prescribe the manner of judicial (or extra-judicial) proceedings in each case following the general laws of the United States.
Then, Section 2 says that citizens of each state have all privileges and immunities under the laws of the states where they are currently staying.
At the same time, a person accused of a crime committed within a state and fleeing justice will be extradited by the justice authorities of any state at the request of the executive government of the state having jurisdiction of the crime.
Section 2 also describes labor relations. In particular, it says that a person held to service or labor in one state cannot escape from this service or labor to another state, and authorities will extradite this person at the request of the party to whom this service or labor was provided.
Then, Section 3 establishes the impossibility of forming new states within the jurisdiction of existing ones, as well as creating new states by merging the existing ones without the consent of the legislative bodies of these states and Congress (which is a federal legislative body). Section 3 also states that Congress has the power to dispose and make decisions regarding territories or other property belonging to the United States.
Finally, Section 4 points out that the United States guarantees a republican form of government to all states and has to protect them against invasion. Additionally, Section 4 says that legislative and executive authorities have to protect citizens against domestic violence.