Radon is a naturally occurring radioactive gas that is tasteless, odorless, and colorless. It comes from the radioactive decay (breakdown) of radium, which comes from the radioactive decay of uranium, both of which are found in at least trace amounts in almost any kind of soil or rock.
Radon enters homes through openings in the foundation floor or walls, wherever the foundation is in contact with the soil. Because it's a gas, radon can travel through the soil, and it generally moves from an area of higher pressure to one of lower pressure. In most cases, the soil is at higher pressure than the house, and if radon is traveling along the foundation, it can be pushed into the lower pressure area through openings such as sump crocks, crawlspaces, space around plumbing or wiring, floor/wall joints, cracks, hollow block walls, or other entry points. Ultimately, tiny or large openings in the foundation floor or walls can act as entry points, and the pressure difference between the soil and the house acts as the driving force that allows radon to enter your home.