The exposure triangle. It seems like a simple concept if you understand some photography. It starts off with "perfect" exposure in the middle, ISO on one corner, shutter speed on another, and aperture on the last. Showing that it you change one it has an effect on the other ones to maintain the "perfect" exposure in the middle. If we change the shutter speed one stop we can either change the ISO 1 stop or the aperture 1 stop, or a combination ISO 1/3 and aperture 2/3 and so forth.
There have been many alliterations of this. I am going to use my own. Imagine cooking an egg. Sunny side up is a "perfect" exposure, with the fire being the aperture, time being the shutter speed, and the pan the ISO. If we have a small fire (small aperture, high f stop), then we would have to either more the pan closer to the fire (increase the ISO) or cook it for longer (keep the shutter open for longer) to cook the "perfect" egg, if we didn't change something then the egg would be under cooked (underexposed) and we risk getting salmonella. Now if we have a large fire (large aperture, small f stop), then we could move the pan, or we could cook the egg for a shorter period of time. If we didn't change something then the egg would be burnt (overexposed). I could go on all day typing each option changing and the others but I am not going to, I think that you probably get it with those two.
After understanding it you can use it to take better pictures that are outside of the "perfect" exposure. Going back to the egg story, some people want their eggs runny, others like them a bit dry. Is either way wrong? No it is a personal preference. The "perfect" exposure doesn't have leeway for those preferences. That is a story for another day though. So go out and shoot.