A neuralyzer is an amnesic device carried by M.I.B. agents and flashes a light that instantly erases memories from unwanted witnesses. The agents themselves are not affected because their dark glasses protect them. People who have just been neuralyzed are left in a susceptible state in which they will be prone to believing whatever the agents told them just happened.
The neuralyzer normally erases memories from a few minutes to hours, but also has settings that allow it to erase days, months or years, which are used whenever an agent is decommissioned or retires.