Aperture, measured in F-stops, determines the size of the opening in the lens. Obviously the size of the opening affects how much light enters the camera, but more what is important from a practical standpoint, is that it determines how much of the image will be in focus - AKA depth of field (DOF).
Understanding which aperture settings to use can be confusing because of the fact that the higher the number, the smaller the aperture, but the greater portion of the image that will be in focus. Just remember that higher aperture numbers mean more of your image will be in focus. And of course, lower numbers mean that less will be in focus (shallow depth of field), which may be what you want when you are taking a portrait, or a flower that you want to stand out from the background.
F/7.1 - Not that low (wide) but I was close to the grass
To help remember how aperture affects DOF, I like to use the analogy of squinting versus opening your eyes wide. If you squint at something, more of your field of vision is in focus, but it’s all slightly blurry. If you open your eyes wide, what you are looking at directly is in focus, but objects in the periphery aren’t. So too with aperture – smaller apertures (high numbers) mean it’s all in focus, whereas, wider apertures (smaller numbers) mean that only what you are looking at directly is in focus. Well, it works for me…