Radicals were a liberal political faction or group during the French Revolution. The French National Assembly established a constitutional monarchy and, in 1791, adopted a new constitution that created a Legislative Assembly. Three factions quickly formed in the new Legislative Assembly, known as the radicals (liberals), moderates (centrists) and Conservatives, similar to those political movements today in the United States.
The three factions sat in different sections of the large assembly hall, with the radicals (liberals) sitting on the left, the moderates sitting in the center, and the conservatives sitting on the right. That gave rise to the left-center-right terminology that we still use today in the United States to describe these three political groups.