panman is certainly right in his post, but I think he might be wanting to know how a magneto generates electricity on its own to create the spark. "Like a lawnmower" was his reference. That probably means he has no idea what an impulse couple is or why it's needed.
If you spin an armature with coils of wire around it inside of a permanent magnet, an electromotive force (voltage) is created by the coil passing through the lines of flux around the magnet*. This voltage created by the magneto is enough to create a spark that jumps across the gap between the electrodes in the spark plugs.
The magneto creates a lower voltage (and less current) at low rpm's like any DC generator. The starter doesn't turn the engine fast enough for the magneto to create a strong spark, so an impulse coupler is used to give it the boost it needs to get the engine running at sufficient RPM.
*a commutator is used to create DC voltage, or as the poles change a negative emf would be created