Digital camera modes allow photographers to control all of the elements of an exposure.
The camera automatically chooses the aperture and shutter speed
(use if you aren’t yet confident in how to set your camera)
Shutter priority (S)
The camera automatically chooses the aperture and you manually set the shutter speed you want
(use for moving objects)
Aperture priority (A)
The camera automatically chooses the shutter speed for you and you manually set the aperture you want
(use to set what you want in focus i.e. if you take a picture of a person you may want to blur the background)
You take control over all camera settings.
Here’s three videos that will help you to understand what each of the modes on your camera does: