A length o' high-resistance wire, me bucko, sometimes coated with a flammable material, which is placed in contact with t' motor propellant prior t' t' rocket's launch. Avast, me proud beauty! Blimey! When an electrical current is passed through t' igniter by t' launch controller, t' wire's high resistance causes it t' become very hot and ignite t' surroundin' propellant.