The Aperture is one of the three legs supporting the stool of photography (Speed and ISO are the other two). It is the size of the hole that lets in light to the camera. Changing the Aperture can affect the speed and ISO and the depth of field (DoF - how much of the image is in focus).
Imagine a real three-legged stool, you can shave a bit off one leg and still sit on it but shave off (or add) a lot, and you have to adjust the other legs as well.
Cameras measure Aperture in fractions (f numbers), and it’s confusing that small numbers represent large holes, lots of light, but a small DoF! A small number is also termed “wide open” and “a fast lens” whereas a large f-number is a small Aperture. Finally, each f-number is half the next higher number or twice the lower number. Are you confused yet?
Here’s a simple tip; Small f numbers = small DoF, Big f numbers = big DoF
In the real world;
Landscapes need a big DoF (big F-number), f9-f16 are common
Portraits, including animals, and birds need a small DoF (Small F-number),
Smooth running water, or waves by using a slow shutter speed (0.25-2 seconds ish) which, with your camera in [A] (aperture) mode can be forced by a large f-number - but you need a tripod.
Sports need a fast shutter speed (1/250 sec or faster), you might be lucky to get this with a small f-number, but you will probably have to go into [M] manual mode and also change the shutter speed, letting the ISO float.
Night shots (large aperture - small f stop) need a high ISO setting (= more noise introduced).
f4.5 – LARGE aperture to give a shallow DoF and isolate the child
f16 – SMALL aperture to give a large DoF and ensure everything is in focus
As with all things in the art of photography, there is a trade-off to consider. Assume you want to take a group of people, so you need a mid-range DoF to get them all focused. Yes, the point of focus will be on the main attraction, the birthday girl, or the celebrating couple, but everybody will want to see themselves, so you decide around f8.
Unfortunately, it’s an evening shot, so you need the aperture to be wide open (remember, small f-number) to let in as much light as possible, but that means Great Aunt Gertrude, at the back of the group, is out of focus!
Typically the FOCUS on the subject (the whole group) is king so stick with f8 (or close) and perhaps bring the shutter speed down a bit (potentially introducing movement blur if you don’t have a tripod), or you can increase ISO (potentially adding noise).
Finally, can you increase the light? Perhaps you have a pop-up flash on your camera or maybe you remembered to pack your electronic flash, or can you move the whole group somewhere brighter.