someone recently asked me this so I thought I would add my reply here to help others out.
Appature is like the pupil in our eye, our eyes automatically adjust to let the right amount of light in.
In low light our pupil is wide open, in bright sunlight it becomes a much smaller circle.
The camera needs us to select the Appature as it can’t do it automatically like our eyes.
So the Appature controls the amount of light we let into the camera, it also determines the depth of field.
The more wide open the Appature, low f number, the narrower the depth of field, the less of the image that is in focus, the more closed the Appature the higher f number, the more depth of field we have, so the more of our image that is in focus.
The shutter speed determines how much light gets into the pupil, a bit like our eyelid, while the eye is open we see, when we close it, it goes dark.
But the shutter speed also determines what will be still in the image. The faster the shutter speed 1/4000 the more we can freeze motion,
this speed would freeze the motion of a racing car, or the slower the shutter speed the more movement is captured. The racing car would be a blur.
Then we also have the camera movement. If you are handholding a camera it is wise to stay above 1/60 sec to avoid camera movement, which is why we use a tripod for lower shutter speeds.
So therefore both Appature and shutter speed can control the amount of light getting to the sensor and by working out the depth of field and the whether we want to freeze the motion or not as to what setting we use.
Generally the bigger Appature (low f number) we have the quicker we can have the shutter speed, and the smaller the Appature (higher f number) the longer the shutter speed we can use.
The more light we get from the Appature the less light we need from the shutter speed and visa versa. I hope this has helped.
I do have a series of e-books on Amazon called “Quick Tips from a Pro Photographer” that take you right from starting out through to more complex topics. Book 1 is called “Starting out” and is only $1.