Background radiation is the low level of ionising radiation that is around us all the time. It is always present, even when there is no radioactive source nearby, and everyone is exposed to it.
It comes from both natural sources and man-made sources. Natural sources include rocks and the air, food and drink, and radiation that arrives from space. Man-made sources include medical procedures such as X-rays, and fallout from nuclear weapons testing and nuclear accidents.
The amount of background radiation is not the same everywhere. It depends on where you live and what work you do, for example in areas with a lot of radioactive rock it is higher, and some jobs raise a person's exposure. When a scientist measures the radiation from a source, they measure the background count first and subtract it from the reading.