Aperture is the technical term for the opening through which light passes to the sensor. The term has been popularized in recent years through a video game, which even uses a camera aperture representation as a logo.
This opening is adjustable, comprised of a circle of overlapping blades which close tighter or open wider depending on your setting. The aperture also depends solely on the lens attached to the camera rather than the camera itself, since the blades are built into the lens, not the camera, which is why you see so many different lens choices, even of the same focal length.
Aperture is also referred to as f-stop for f/(number). The “F” stands for the “Focal” part of “Focal Length.”
There is a lot of confusion when it comes to these numbers, because of one simple thing: small numbers are bigger than large numbers. I know, it seems counterintuitive, but hear me out. f/1.4 is a lot larger than f/16. You would “stop up” from f/1.8 to f/1.4. This refers more to the size of the aperture pupil than the size of the number. The smaller the number, the wider the pupil becomes. Still confusing? It was for me, too. Think about those blades I mentioned earlier. As the opening grows wider, the blades contract. Smaller blades, smaller f-number, larger aperture. That’s a good way to remember that smaller numbers count for more.
The aperture is actually the setting that affects the overall appearance of the image the most, brightness notwithstanding. This is where we also come to Depth-of-Field, because the wider you open your aperture, the brighter the image becomes, and the more blurry everything outside of the subject becomes.
Depth-of-Field is the portion of space that is in-focus for your photograph. You know the funny pictures of people “holding” the Hollywood sign, or “leaning” on the Leaning Tower of Pisa? The whole thing is in focus. That’s a very high depth-of-field. For a high depth-of-field, there must be a lot of light, because the aperture pupil becomes much smaller and allows less light to reach the sensor.
If you hear someone referring to a “shallow depth-of-field” they mean that they want the aperture opened to a high number, like f/1.4, they’re allowing a lot of light into the camera through the wide aperture pupil, and that anything in front of, or behind, the narrow portion of space they are focusing on will be blurry.
Summary: Aperture = opening for light. Found in lens, not camera. Smaller f/number = larger (worth) number = contracted aperture blades = wider aperture pupil = more light = shallow depth-of-field.