Sometimes, It's Good to Make it a Priority
Mastering exposure can be challenging. Sure, you can leave everything up to your camera on automatic, but what's the fun in that? But while I usually prefer to take complete control with manual exposure, sometimes aperture priority or shutter priority is a better option.
Virtually all modern cameras come with an automatic mode. Indeed, I'll wager this is so common, at least a few beginning photographers don't yet realize more options exist. On automatic, a camera will take care of the messy business of setting the exposure for you. You don't have to consider selecting an appropriate aperture and shutter speed any more than you give thought regarding the dilation of your pupils when you look at the world without a camera. It's automatic. And as a third variable, a digital camera can even manage the ISO setting for you as well. But as tempting as this may seem, even the best camera isn't a mind reader and isn't likely to make the same choices you might consider every time.
There's more than one solution to the exposure problem, as described by something known as the rule of reciprocity. Add or subtract a stop to one variable, and you will maintain the same exposure if you also deduct or add a stop to another. So, for example, a shot at f/8 aperture for 1/500 second will result in the same exposure as one taken at f/5.6 for 1/1000 second. But one with the smaller aperture will show a greater depth of field, while the faster shutter speed will be less apt to show motion blur. If you don't want your camera to decide which version it prefers, you're going to have to take control yourself.
Full manual mode gives you the most control. The camera meter will still let you know how bright something is but will leave it to you to set all variables of the exposure triangle yourself. You get to pick the ISO, aperture, and shutter speed as you see fit. The meter will tell you if the result will be over or underexposed, but you will be free to pay as much attention to its input as you want. It can take a while to master this much power, but the creative expression possible through manual exposure is hard to deny. Or at least you won't be able to blame your camera for the results anymore. You get all the credit when things come out well, and you get all the blame when they don't.
There isn't always time to exercise such control properly. In a perfect world, I'd shoot everything on manual exposure. But in a pinch, I'm glad to have the option of aperture priority and shutter priority.
When set to aperture priority, you get to pick the aperture yourself, and your camera will attempt to balance that choice by selecting the shutter speed for you. Beyond its effect on exposure, aperture directly influences depth of field, so you're in control. Ratchet the aperture down to a small hole, and you can create shots with everything sharp from the foreground to the horizon. Open wide, and just a thin slice of nature will be in focus.
Shutter priority is the reverse. You can control the time the shutter remains open while the camera keeps the exposure balanced by picking an aperture to match through reciprocity. The slower the shutter speed, the more subject motion will show in your image. If you're shooting a waterfall, you'll end up with a lovely, silky rendering. If you're shooting a field of flowers, you'll wind up with a blurred mess. But the choice is yours, and the camera will keep tabs on the exposure, so you don't have to worry about everything yourself.
When you shoot on automatic, aperture priority, or shutter priority mode, you can still influence exposure to create a high-key or low-key rendering using the exposure compensation control. Without it, your camera will attempt to render everything as "medium toned," the premise being that most scenes work out to average on the whole. Exposure compensation lets you bias this target to be something other than medium without worrying about all the specific settings needed to achieve it. But there are a couple of things to be careful of with exposure compensation. First, it does nothing in manual exposure mode other than confuse you. You still have to set each exposure variable manually. And regardless of what exposure mode you work with, remember to reset the exposure compensation after each subject. It will still affect the meter reading if you know it's turned on.
I love aperture and shutter priority when the lighting conditions are changing rapidly. Light levels change drastically as the sun passes over the horizon. After struggling to keep up with things shooting with manual exposure, I finally learned to appreciate the benefits of the priority modes. Both let me spend the precious time I have on what matters most, using the camera logic as an assistant. Back when I was learning about exposure, it was a thing that real photographers shot manual and that the other modes were there for those who couldn't cut it. Instead, I'd suggest that real photographers know how to best use the tools at their disposal. And this includes all the tools, including the knob to select exposure mode. Results are what counts, no matter how you get there. I still shoot on manual whenever I reasonably can, but there's no shame in the priority modes. And yes, there are even times when automatic might be the best. If I'm shooting a birthday party, the lighting rarely is that challenging, and I want to enjoy the occasion, not worry about camera settings.
In all four modes, it's best to shoot in RAW capture if you have any doubt about exposure to provide greater latitude later in Lightroom or Photoshop. It's almost like having a second exposure compensation control after the fact.