Actor sequitur forum rei is a Latin term that means "the plaintiff must follow the forum of the thing in dispute." Essentially, in the present day, this phrase means that plaintiffs should initiate their lawsuits in the region where the matter took place. This is because laws may change from region to region.
Sometimes, plaintiffs want to file suits in areas outside of the region where the incident took place, or where the issue has the most relevance. This could be because the plaintiff lives far away from the region of interest, or because he or she prefers a court in another region, etc. However, actor sequitur forum rei claims that plaintiffs should file lawsuits in the region of relevance. So, for example, a man in New York City should not file a lawsuit in Toronto if he is suing another New York City resident about a private nuisance in New York City.