You know when you see those cool pictures when the subject is in focus but the background is blurred out and you wonder, “How they do that?”. I am going to attempt to explain this simple stupid.
A picture is dependent on light right? Well the aperture is was determines the amount of light that the image receives. For example if you take a bowl and turn the faucet on for 1 second with a small stream you get less water then if you open it for 1 second full throttle, right?
Okay, so if you want to blur the background behind your subject, you want the most amount of water or light as possible, if you want the background to be in focus along with your subject, you want a small amount of light or water.
Now hold on to your hats because this is the part that gets confusing, you ready? The camera measures the amount of light by F-stop, which is the measurement of the diameter of the opening that is allowing in the light. If you want a lot of light/water which blurs your background, you want a small F-stop number like 2.8, 4.0, 5.6. If you want a small amount of light which puts your background in focus, you want a large F-stop number like 8, 11, 22 and so on. This is also refered to as depth of field. Which is basically the how much of the depth of the picture is in focus. I suggest reading this portion 15 times until you get it.
Here is a diagram to help you: