The camera lens acts like a funnel in that it gathers light. The aperture is a mechanically operated, circular opening. Its job is to adjust the working diameter of the lens. The greater the lens diameter, the more light gathered. When the light is feeble, we set a large aperture diameter. When the vista is brilliantly lit, we set a tiny aperture diameter. Additionally, the diameter of the aperture is often used to cause different imaging effects. This is possible because the aperture diameter is intertwined with shutter speed and the sensitivity setting (ISO) of the camera.

Now to answer your question: You can draw an imaginary line from the subject to the lens. You can continue this line as it changes direction when it passes through the lens until it bumps into the light sensitive surface of the image sensor or film. Now that you have drawn this line, draw another from some other part of the vista. Now continue drawing. You will find that you must draw zillions of lines. In other words, you must draw lines from every point on the subject to the top of the lens, to the center of the lens, to the left side of the lens, to the right side of the lens. In other words, light from a single point on the subject hits everywhere on the lens. All these rays pass through and then play on the film or digital chip. If you interpose a circular aperture, you stop some of the light rays that hit the boundaries of the lens. But you do not stop the remainder. Those that get though project an image on the film or chip. However, you have cut down the volume of light that can get though, and the image dims as you close down aperture diameter.

The mechanical aperture can be inside the lens which consists of many glass lenses. The aperture can be placed before the lens, the aperture can be behind the lens. We place the aperture inside to protect the delicate thin metal blades, and because the position will effect optical performance.