A more thorough explanation:

The collateral order doctrine is an exception to the general rule against allowing interlocutory appeals. Interlocutory appeals are appeals on a temporary order issued during the course of litigation. As a general rule, only final judgments are appealable. Nonetheless, some interlocutory decisions act as final judgments to certain rights. Therefore, interlocutory decisions are appealable under the collateral order doctrine if they fulfill three conditions:

1. The interlocutory decision conclusively determined the disputed question
2. The disputed question is important and entirely separable from the merits of the action
3. The interlocutory decision is effectively unappealable after a final judgment is handed down

For example, in Cohen v. Beneficial Loan Co., an interlocutory decision regarding whether a New Jersey statute which required parties to offer a security interest before trial is applicable in federal court was subject to the collateral order doctrine and could therefore be appealed before the trial’s conclusion.

Another example could be a decision on whether a defendant's confession was obtained illegally. If the court determines that the confession was obtained illegally, it may be subject to the collateral order doctrine and could be appealed before the trial's conclusion.

These examples illustrate how the collateral order doctrine allows for certain interlocutory decisions to be appealed before the conclusion of a trial if they meet the three conditions outlined above.