The Kelo case was one of the biggest in recent years in terms of the impact it had on charges of judicial activism. It made many people very angry -- enough so that many states passed laws prohibiting actions of the sort that the Supreme Court had endorsed in its decision in this case.
Under the doctrine of eminent domain, a government may force a private citizen to sell his or her property to the government. It may only do so when the property is to be used for some public use. In the past, the term "public use" had been taken to mean things like freeways -- things that are owned by the government.
In the Kelo case, however, the government was going to buy the property (by eminent domain) and allow it to be turned into a privately-owned shopping mall. The idea was that the mall was good for the community and therefore a public use.
This worried people greatly about activist judges because this doctrine could end up allowing the government to take property from anyone and give it to anyone else simply by arguing that it was good for the community to do so.
This is why the case was so relevant to the charge that activist judges (here they are activist because they are expanding the meaning of "public use") are hurting the political system.