Photography is all about light.
Controlling aperture permits YOU to control the amount of light and there by controlling depth of field (how sharp or blurry the photo is).
I suggest (if your camera has this option) you shoot in aperture priority (usually AP or A on the dial). You can control the aperture and it will automatically select a shutter speed.
Basically when it comes to aperture, the LOWER the number, the MORE light is let through and the more SHALLOW (or fuzzy) the photo/background becomes. The HIGHER the number, LESS light is let through and MORE of the photo is in focus.
As you can see in this progression, starting with the first photo, barely anything is in focus, the background is just a wash of color and only a couple petals are in focus, by the time you reach the last photo, EVERYTHING is in focus including the window screen which is now well defined.
When you hear someone saying, “I shot wide open”, usually they are referring to using a LOWER number aperture, like f/1.8 for example, which lets MORE light through the lens and creates SHALLOW depth of field.
Now remember how I mentioned that photography is all about light? If you are in a brightly lit area, direct sun for example, you may NOT want to shoot “wide open”. Doing so again lets the maximum amount of light through the lens and in that bright scene it could be TOO much light and overexpose (make the photo too bright/white/washed out) your photo.
Eventually I will do a really basic shutter explanation, ISO, and how to put aperture and shutter speed together.
This post is over a year old which means the content may be outdated or no longer accurate.