There is always a small amount of radiation around us, even when no radioactive source is nearby. This is called background radiation. It comes from natural sources on Earth and from space, and also from a few human-made sources.
On Earth, some rocks contain radioactive isotopes. Granite is an example. These rocks give out radiation, and they also release radon, a radioactive gas, which can build up inside buildings. Radioactive isotopes are also found in the soil, in food and in drink, so our own bodies contain tiny amounts of them.
From space, cosmic rays reach the Earth. These are high-energy particles that come mainly from the Sun and from other stars. The atmosphere absorbs many of them, so people at high altitude, such as in aeroplanes, receive more. Human-made sources, such as nuclear weapons testing and nuclear accidents, add a small amount to the total.