The Reign of Terror was a period of time during the French Revolution when officials would randomly execute people who were thought to be a threat to their plans. The Reign of Terror was ran by a group called the "Committee of Public Safety." The group ruled France when it was going through a "national emergency," such as the French Revolution, and was considered to be a dictatorship.
The Committee of Public Safety formed on April 6, 1793 to preserve the reform during the French Revolution. By September 6 of the same year the group took its final form. They had twelve members and their main member was said to be a man named Maximilien Robespierre.
The group's prime objective was to eliminate anyone who would get in the way of the revolutionaries. Like in Tale of Two Cities, people that came into France were commonly though of as traitors and were executed immediately.
Also, the group tried to form a militia of the people in France that were fed up with the aristocracy. They also had to provide food supplies to people and the armies.