A radioactive gas which is naturally occurring. There's radioactive material pretty much everywhere underground. Some places more than others, but it's basically everywhere. These are natural radioactive deposits, and they're usually not concentrated enough for mining or anything, it's basically trace amounts, more or less. It's usually deep enough that there's no radiation making it to the surface and no danger, however, when it decays it releases radon gas, which still isn't really dangerous unless it reaches high concentration. Since it's a gas, it leaks up to the surface and if you have a basement especially, it can seep through into the basement, and if your home is fairly well sealed, it will accumulate.

If enough accumulates, it increases your risk of certain types of cancer.

I'll be clear, the danger of radon is in long term exposure to relatively "high" concentrations of the gas. If you monitor it and take measures to reduce the levels actively and on an ongoing basis, you're probably fine, since the levels are, on average, below the danger zone. If you're consistently above danger levels, then it's possible that you could get cancer as a result.

If you don't have a basement, you're basically immune to having a concern. Unless your ground floor is on a concrete slab on the ground or something, which might pose a risk of it seeping in.

Radon has a half-life of a few weeks, so to reach the accumulation and duration requirements for danger, it would need to be a fairly consistent flow into the home, with little to no ability for it to escape into the atmosphere outside.

There's a few ways to mitigate it, including radon vents under the home, or a ventilation system to exchange the homes air volume outside, something like a whole home ERV may be enough to remove/reduce it.

Highest risk would be those in older homes which have a basement, with more modern "high efficiency" insulation techniques, which generally cause less leakage of inside air to outside air (and vice versa), which are almost consistently sealed with heat/AC on.

I'm at risk since I believe this home is between 20-40 years old, so it was built on or before the 2000's, when they had pretty good home insulation, but didn't have the option of an energy recovery ventilator at the time. The windows have been upgraded which sealed the house further. We have a reasonably sized basement, and no ERV at this time. I'm planning on adding a few ERV units and a radon ventilation system (either under-home or in the basement) to vent it safely away from the house. The ERV units are mainly for CO2, but they should help with radon as well, and having a dedicated radon vent system in the basement should drop levels well into the safe zone.

If your house has a crawlspace underneath instead of a basement, one which is not in the insulated envelope of the home, then there's no risk, since the crawlspace is where radon would have an opportunity to accumulate, but it's vented directly with the outside atmosphere, giving the gas an unrestricted path away from your living space. Anything built above ground level, such as condos or apartments have basically zero risk, unless they share ventilation with an underground area, which most do not.

I picked up an airthings radon detector and levels fluctuate wildly for us. From 46 Bq/m3 to a little less than 600 Bq/m3. "Safe" levels are below approximately 100 Bq/m3, and "danger" levels are above 150 Bq/m3. Between those is the "warning" levels. My meter is in the basement, not my living areas, so it's showing the worst possible numbers, which may not be representative of levels in living/sleeping areas, since the basement has no HVAC vents/returns, and would be fairly stagnant. The basement is also the last place in my home to be ventilated, since we tend to open windows in living areas, leaving any windows in the basement closed. So I expect the numbers in the rest of my home to be less than whatever the meter is showing at any given time. AFAIK, radon is generally heavier than air as well (though, not by a lot), so it should also settle in the basement, driving the reading up even further.

Since it takes years of danger level exposure to increase the likelihood of cancer, and other factors like CO2 can concentrate to toxic levels in a matter of days, even in a fairly large, but well sealed home, I'm more focused on CO2. Our CO2 levels when the house is sealed, can go above 2000 ppm, which is normally when I feel like I need to take action, usually it takes about 30hrs or so to go from atmospheric CO2 levels (when the home is open, around/below 450ppm) to 1800+ but it can happen more quickly if we have more people in the home. Typically we have four adults, and five pets. I imagine we would hit toxic levels of CO2 within a week of having the home sealed with typical occupancy (around 4000-5000 ppm is when it gets dangerous).

I'm acutely aware of my dangers, and I'm currently trying to save money to deal with both issues permanently.