Res judicata is a legal principle that prevents the same case from being tried again once it has been decided by a competent court. This doctrine ensures that once a final judgment is made, the parties involved cannot relitigate the same issue, promoting judicial efficiency and finality in legal disputes. The purpose of res judicata is to protect parties from the burden of multiple lawsuits and to uphold the integrity of the judicial system. It applies to both civil and criminal cases, ensuring that legal decisions are respected and not repeatedly challenged in different courts.