A more thorough explanation:

The Rooker-Feldman doctrine is a rule that says a federal court cannot consider claims that have already been decided by a state court or claims that are closely related to an earlier state-court decision. This means that if you lose a case in state court, you cannot go to federal court and ask them to review the state court's decision.

For example, let's say you were in a car accident and sued the other driver in state court. The state court ruled in favor of the other driver, and you want to appeal the decision. However, you cannot go to federal court and ask them to review the state court's decision. This is because the Rooker-Feldman doctrine prevents federal courts from acting as an appellate court for state court decisions.

Another example is if you were evicted from your apartment by a state court. You cannot go to federal court and ask them to review the eviction decision. This is because the Rooker-Feldman doctrine prevents federal courts from reviewing state court decisions.

In summary, the Rooker-Feldman doctrine prevents federal courts from reviewing state court decisions or claims that are closely related to those decisions. This is to prevent people from using federal courts as an appellate court for state court decisions.