Radon comes from the natural breakdown of uranium in soils and rocks.  It seeps out of the ground, into basements and moves to other areas of a building or home.
Radon gas can move through small spaces in the soil and rock upon which a house is built. It can seep into a home through dirt floors, cracks in concrete, sumps, joints, basement drains, etc. Concrete block walls are particularly porous to radon.