The exposure triangle is probably not new to you, the point is simply that the aperture, the shutter speed and the ISO controls what level of exposure your picture will get. Changing one of the three in upwards direction will require one (or both) of the other two to drop accordingly to maintain the same exposure. And the other way around. In (fully) manual mode, you set all 3 values.
Aperture is the iris of the lens. The more light you let in, i.e. the more open the aperture is, the more exposed your picture will get. The confusing part is that the aperture is measured in f-stop numbers, and these are “upside down”, meaning that a low F-stop number means the lens is letting in a lot of light.
Shutter speed is more intuitive, if you ask me. The longer the shutter speed the more light is allowed to hit the sensor. A fast shutter speed is great if you want to avoid camera shake (you move the camera while the shutter is open) or motion blur (the subject moves while the shutter is open). But fast shutters lets in only a little light, so a balance it to be found.
Finally ISO is how sensitive your sensor appears to be. I say appears to be, as it technically is a gain that is applied to the base sensitivity of your sensor, but don’t focus too much on this part. All we need to know is that higher ISO means more sensitive to light. There is no free lunch in photography – so the price to pay for high ISO is grain. So we want to keep the ISO at bay. Modern cameras can ramp up the ISO with impressive results, but for now let’s just say that beyond 1600 you need to be more cautious.
