Res judicata can be translated as “a matter that has already been judged.” This idea is important to civil litigation because it plays something of the same role that the prohibition against double jeopardy plays in the criminal system. The notion of res judicata holds that a thing (case or issue) that has already been judged cannot be judged again once it has been appealed as far as it can be.
This rule is important for two main reasons. First, it is important to the efficient running of the court system. If certain questions could be litigated over and over, the court system would be even more clogged than it already is. The second reason has more to do with justice than with efficiency. Here, res judicata is meant to prevent a defendant from harm. Without this rule, a plaintiff could sue a defendant multiple times, winning multiple awards for the same harm. Without the rule, a plaintiff who loses could continue to sue the defendant, causing a tremendous amount of harm in terms of legal fees and time lost.
For these reasons, this is an important concept in civil litigation.