Aircraft ignition system

Aircraft ignition systems function is to provide an electrical spark to ignite the fuel/air mixture in the cylinders. This is very similar to a modern car. Modern airplane engines are required by to have a dual ignition system. This means that there are, two separate magnetos to supply the electric current to the two spark plugs contained in each cylinder. This dual ignition system

Dual ignition used in aircraft engines

Dual ignition is most commonly employed on aero engines though it is sometimes found on cars and motorcycles. Dual ignition provides two advantages that are readily seen. These are redundancy in the event of in-flight failure of one ignition system. Meaning it is safer to have a dual ignition as it is less likely to fail then a single ignition system. While also having a more efficient burning of the fuel-air mixture while in the combustion chamber.

Magnetos on an airplane

An aircraft uses a magneto to produce a high voltage that is used to fire off the aircraft spark plugs. Aircraft magnetos are used in piston aircraft engines and known for their simplicity and reliability. It is an engine-driven electrical generator that utilizes magnets and coils to run the pistons.
These use a very strong magnet rotating inside a coil. The rotating magnetic field generates a voltage in the coil which is transformed to a higher voltage by a secondary coil with much more windings than the primary coil. A breaker contacts in the primary coil circuit interrupts the flowing current and this interruption causes the magnetic field to collapse thereby generating a very high peak voltage in the secondary coil. This peak voltage is then conducted to the correct spark plugs by the distributor and high voltage leads.
Two magnetos are connected in such a way that one drives the top spark plugs and the other magnetos operates the bottom plugs on the engine. If you hear left and right magneto this really means top and bottom spark plugs. The magnetos generate power independently of the aircraft electrical system, so that in the event of flat battery during the flight the engine will keep running. This is great for obvious reasons.