Whats aperture for?

So ive recently gotten into photography and im watching a couple videos on how to handle iso, shutter speed and aperture. The thing is that people keep saying that you can adjust your aperture to determine what stays in focus but when i mess with the aperture all it does is it makes the image darker. However when i adjust my lens thats the thing that changes what stays in focus. So am i using aperture wrong? cause i dont see the point in changing the size of the aperture any smaller.