Definition - What does Issue Preclusion mean?
Issue preclusion means that a particular issue, claim or question of fact that has already been handled in court cannot be relitigated. If the particular issue has already been litigated, and a judgment has been reached, then the issue is considered to be closed. Another trial on the exact same subject matter is not permitted.
Justipedia explains Issue Preclusion
If issue preclusion was not included in law, then a party could continue to sue another party over the same exact claim time and time again. This would waste endless amounts of the court's resources. For example, imagine a man hired a contracting company to do $10,000 worth of work, and the comany simply took the money and did none of the work. It would be completely unnecessary for the man to sue the company again if he already sued them once and won the money back. So, if he tried to do so, the case would be thrown out due to issue preclusion.