A technical noun as defined in the dictionary as being an opening, gap, or hole.
Quickly put, aperture is amount of space you are providing to your camera to let light in.
This is achieved by opening the aperture blades within the lens. You do this by changing the f-stop on your lens – either via turning the aperture ring on the lens, or by using the aperture dial on your camera.
For example, a lens may range from f0.95 to f45 with many other stops in between – with (in the below pictured example) f1.8 being the largest in terms of openings – meaning more light will shine through into the camera, and f11 being the smallest, meaning less light, but more in focus on a technically correct exposure.
Why would you want to choose how much light comes in? Well, in darker scenes, you may want to be able to let more light in to get a faster shutter speed, which can eliminate blur. You may want to create beautiful bokeh in the photograph, meaning the background is nicely blurred out with your subject the main focal point.
Depth of field is something to think about too. What do you want in focus and what do you want out of focus?
You can set your camera to Aperture Priority mode, meaning you choose the aperture you want and the camera will determine the rest.
I would say that street photography is mostly shot at f5.6 or f8 – This is due to the photographer wanting not only the subject in focus, but some of the surroundings too, so you can see what is causing the subject to be the main focal point.
Landscape photographers may swear by f11, f16, f22 etc because they want to expand the depth of field beyond what’s near to them, and maintain tack sharpness throughout the image.
Portraits may be taken at f1.4 or f1.8 so as to provide a blurred background from the subject.
Of course, photography is purely subjective to what the photographer wants to show the viewer. The only technical aspects are just choices. No right or wrong.