Stops: the unit that ties aperture, shutter speed and ISO together
Beginners tend to memorise aperture, shutter speed and ISO as three separate topics, and then feel lost when changing one of them makes the picture suddenly go dark. What links them is a shared unit called the stop. One stop means twice the light or half of it. Once that idea settles in, the three settings start to behave like three taps that can stand in for each other.
On the aperture ring, the full stops run f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16 and f/22, and each step to the right halves the light. Shutter speeds follow the same logic: 1/1000, 1/500, 1/250, 1/125, 1/60, 1/30 and 1/15 of a second. ISO 100, 200, 400, 800, 1600 and 3200 each double the camera's sensitivity to light compared with the value before.
The easiest way to practise trading stops is beside a window. Photograph one object at f/4 and 1/250 s, then switch to f/5.6 and 1/125 s. Brightness stays the same, because the stop you lost at the aperture came back through the shutter. Repeat with three or four more pairs and write each one down. When you can predict the next matching pair without checking the screen, the core logic of exposure is in place.
A stop means double or half the light, and that same unit applies to the aperture, the shutter and the ISO dial.