The simplest way to think of this is the brightness of the image. When an image is underexposed, it is too dark, when it is overexposed it is too light. The tricky bit is when there are multiple very bright and very dark bits to the image, but that just comes with experience.
Exposure is controlled by 3 factors, each with an effect on the exposure and a secondary effect often referred to in the exposure triangle as seen below:
This is how big the hole is in the lens and designated by 'f'. Confusingly, the smaller the number, the bigger the hole. It can range anywhere between F/0.95 to F/32.
The above images demonstrate this perfectly. The first was taken at F/1.4, is focused on the lens cap, and the camera behind is blurred.
The second image was taken at F/4, it is darker, and the camera is less blurred.
The third image was taken at F/8 and shows very little blurring of the camera behind, but is very dark as the aperture is much smaller.
So generally if you are taking portraits, to achieve a good, shallow depth of field, you need a lens with a wide aperture (I would say F2.8 at absolute minimum, although there are other ways it can be achieved.)
When you buy a lens, generally the more expensive lenses have a bigger aperture, and the lens name indicates what it's widest aperture is. E.g. Nikon 50mm F/1.4 has max aperture of 1.4 (which is probably the biggest hole that is 'affordable')
The shutter is the click you hear when you take a photo, it is literally a flap that opens to let the light in onto the sensor. The shutter speed, is how long the flap stays open. It is designated as a number which represents fractions of a second e.g.e a shutter speed of 100 = 1/100th of a second, 10" = 10 seconds.
As you can work out, the longer it is open, the more light enters onto the sensor making it brighter. But also if it stays open, motion of what happens ends up blurring the images, which is some cases is the desired outcome. That is what long exposure photography is.
With regards to the above images, the first image has a shorter/faster shutter speed and the scond has a longer/slower shutter speed. The long exposure on the right image caused the motion of the water to form a calm ethereal and fail to look like water anymore. The reason the right image is not darker, is because it had been taken when conditions were much darker as the sun had already set.
The last part of the triangle. ISO back in the day was the sensitivity of the film. So high ISO film was more sensitive to light and could be used in low light situations. Now it is the adjustable sensitivity of the sensor and is represented in exponential increases from 100 anywhere to 512000 or higher. The normal range people usually is between 100 and 6400.
The unwanted effect is that as you increase ISO, the image begins to develop 'noise'. These are essentially dots that appear on your image to make it look less sharp. During the early days of digitial photography this was a real problem and most people didn't go above ISO 1600, however most modern cameras have good noise performance, especially the higher end models.
In order to get the right exposure, these 3 parts have to be balanced. If you shoot on Auto or P mode (P mode is basically auto with a little more control), the camera does it for you. But the camera makes mistakes and in order to fully appreciate a good exposure, you have to be able to manage it yourself. To do so requires shooting in full manual mode or M mode and this is what I would recommend. Then there are dials on the camera to adjust.
There are modes where you can just change the aperture, and the camera automatically adjusts the rest or do the same with shutter speed - these are aperture priority (A mode) and shutter priority (S mode), some people like to use these when starting out.
So that's it for the camera part of exposure. The rest of the exposure comes from the post processing of photos, but that's an entirely long topic for another day. Shoot in RAW format instead of JPEG if you want more freedom in editing and then just go out to practice. Most other things about cameras come with time.
Now for real tricky bit - composition, it's got it's own dedicated page.
Cameras are often set out differently with some having dedicated dials to shutter speed and the aperture control on the lens. This shouldn't intimidate you, it just becomes a matter of turning the dials and figure out what works for you.