The radial engine is a reciprocating type internal combustion engine configuration in which the cylinders "radiate" outward from a central crankcase like the spokes of a wheel. The radial configuration was commonly used for aircraft engines at turn of previous century.
If you want to know how the radial engines work, have a look at this video:
Radial engines have been used not only in aeroplaces but also in tanks, combat cars, self propelled howitzers so it was quite natural progression that someone would come up with an idea of using these engines in Motorcycles.