A person put in place(hence 'Plant') to divert things. Plants are used to divert movements, plans, plots, or whatever an opposing force wants to stop. Using a plant is a clever way of destroying an opposing movment without seeming suspicious or have to get you hands dirty. Sometimes plants are sent my one group to an enemy group to act as if he/she is part of the enemy group and try to gain influence just to mess the group up, or make them look bad. If democrats wanted to make republicans bad, they could use a plant that fakes like a republican and tries to make republicans look bad, and in turn make the democrats look better, even though the plant is playing a part prompted by the democrats. Or say if you playing a competitive game, and the opposite team sends in a plant to your team, acting as a friend, but he/she is really there to mess your team up and divert it so that the other team(who placed the plant) wins.
Figures that Jason was a plant, it seemed as if his whole point was to disband our protest from the beginning.