Before reading further, I strongly encourage you to check out this post on the Exposure Triangle. I can't possible explain it any better, and it's critical to have an understanding of how ISO, Aperture and Shutter Speed work together before diving deeper into any of them.
Have you read it? Good.
On the back of your camera, you'll see a setting on your screen that begins with a 'f'. This is the aperture, or f/stop. This setting determines how much light is let into your camera and how much of your photo will be in focus. The higher the number, the more of your photo is clear. The lower the number, the more blur you'll achieve.
Aperture controls the opening in your lens. A wider aperture (lower number) lets in more light, but can also cause you to miss focus if the depth of field is too shallow. A narrower aperture (higher number) will ensure that more of your photo is in focus, but can cause your shutter speed to slow down and/or raise your ISO which can result in a grainy, noisy photo. Generally, portraits are shot with a wider aperture, while landscapes require a narrow aperture to ensure that that more of the scene is in focus. The key is finding a well-lit space to take your photo to ensure that you have the desired depth of field without sacrificing clear focus, free of motion blur, or noiseless, grain-free images.
If you asked me the single best thing you can do to improve your photography and really learn how the exposure triangle works, I would tell you to practice with a 50mm prime lens ('nifty fifty') to really understand aperture.
A prime lens doesn't zoom. To get closer or further away, you move your feet. Some may find that a bit strange, especially as new users are probably using the 18-55 lens that came with their cameras (called the 'kit' lens.) A kit lens can be great...it has a nice range allowing you to get a wide landscape or large crowd shot, while also allowing you to zoom in for closer photos. It's pretty versatile. The one thing your kit lens won't really allow you to do is get that beautiful, blurred background (called 'bokeh') that you've seen in so many photos. Why is this? With a kit lens, the lowest f/stop is generally between 4.0 and 5.6. You can shoot at f/4 when you are zoomed out (18mm) and f/5.6 when you're zoomed in (55mm.) These settings don't isolate the subject and blur the background as well as other, wide aperture lenses. The narrower aperture is not the main issue, however. The issue with the kit lens is that the aperture value is variable. It will change, depending on your focal length. This is not a bad thing if you're taking snapshots or not concerned with manipulating the settings for more creativity. It is a drawback if you're trying to learn to shoot in manual mode or wanting use your camera in lower light.
A nifty fifty can open up (and stay) as wide as f/1.8. It doesn't change unless you manually change it. That's a pretty big difference from a kit lens. Let me show you...
This shot was taken at f/5.6, which is often the widest a kit lens will open:
Notice that Batman is sharp, but the other figures are somewhat blurry? This can be achieved with a kit lens. It's certainly blurry enough to allow for some creative shooting. However...
Look at the results using a 50mm lens, with the aperture set to 1.8:
Notice that the figures in the back are almost unrecognizable. This can be quite beneficial in a cluttered environment, or one where the background isn't attractive. It allows you to isolate the subject nicely. You can tell just how shallow a depth of field will be captured by the very thin stripe of focused space on the table. Batman and directly next to him will be in focus, but nothing else.
As you read in the post I linked above, every time you adjust one setting, you likely need to adjust the others (while shooting in manual.) If you are in AV mode, your camera will do the work for you, but not always the way you want. I used AV mode for the examples to show how the camera may decide to adjust your settings.
The camera determined the settings as follows:
Aperture Shutter Speed ISO
1) 5.6 1/320 5000
2) 1.8 1/320 640
In the first photo, with a narrower aperture of 5.6, the camera decided to compensate for less light coming in by raising the ISO. 5000 is quite high, and can result in a very messy, grainy photo. A better option would have been to reduce the shutter speed (again, the longer the shutter is open, the more light that comes into the camera.) With a still object, you can reduce your shutter speed considerably. A good rule of thumb is 2x your focal length. Since I used a 50mm focal length, I would try to stay above 1/100 with a non-moving subject. I would then be able to reduce the ISO to a much more manageable level.
In the second photo, the camera did a decent job with the settings. I would have reduced the shutter speed a bit to also reduce the ISO. I try to keep my ISO no higher than 400 unless absolutely necessary - night, low light, etc. However, the settings will result in a clean, well-exposed, noiseless photo.
Make sense? You have to remember that changing one setting affects the others. Using a wider aperture, which allows more light into your camera, will require your to raise your shutter speed and/or lower your ISO to avoid a too-bright photo. Using a narrow aperture, for a landscape or large group, will reduce the amount of light that your camera lets in. You need to compensate by slowing your shutter speed and/or raising ISO. My preference is to try to adjust aperture and shutter speed, and only adjust ISO if absolutely necessary.
Next up...Shutter Speed!