One of the most fundamental units of measurement in photography is the stop. What is a “stop”? Beats me. What does a “stop” mean? That’s much easier. A stop up is a doubling of light reaching the sensor, a stop down is a halving of the light reaching the sensor. So what? So everything!
If you are comparing lenses you often compare their aperture, the difference in aperture is measured in stops. If I want to take a milky way shot and have only an f/4 lens but are looking at a beautiful f/2.8 lens to buy, how do you quantify the difference? Stops! The difference between f/2.8 and f/4 is exactly 1 stop, or twice as much light. That means I can either get a brighter milky way image or I can reduce my ISO by half and get the same exposure as with the f/4 lens.
Remember the exposure “equation”… “shutter speed + aperture + ISO = exposure”. Every exposure is a balance between the three components: shutter speed, aperture, and ISO. And all of this is measured in stops. If my exposure is 1/10s at f/4 and ISO 1600 and I want to switch to ISO 800 I have to adjust my shutter speed by half as well to 1/5s in order to get the same exposure. Get it? Its easy when it comes to shutter speed and ISO. Aperture is a little more tricky. The whole stops of aperture go 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22, 32 with each step up or down being a whole stop. So going from f/2.8 to f/4 is 1 stop while going from f/4 to f/8 is 2 stops. So an example: my exposure is dead on at 1/100s at f/4 and ISO 400 but I want more depth of field so I increase to f/8. I will have to make up 2 stops of light loss so I can lower my shutter speed to 1/25s (1/25s at f/8 and ISO 400) to keep the same overall exposure.
The exposure for “View From Heaven” was 1/250s at f/8 and ISO 100. The shot would look exactly the same if my exposure was 1/500 at f/5.6 and ISO 100. Exactly the same minus the change in depth of field by going from f/8 to f/5.6 though in this shot it probably would not have made a difference. But seriously, stop worrying and get out and shoot!