Once you understand the basics of the exposure triangle and the three variable settings that go into it, you can start to venture away from Auto setting and begin experimenting with manual. Programme (P) is a full auto setting in which the camera still controls the exposure based on the scene. It differs from the default Auto setting in that it unlocks more features within your camera's menu system.

Aperture Priority (A or Av) is a setting in which you set the desired aperture value and the camera decides the shutter speed and ISO to provide the correct exposure. You might use this setting if you want a specific depth of field in your image. Keep in mind that as the camera will be choosing the shutter speed, it could be fast or slow and give some motion blur.

Shutter Priority (Tv or S) is a setting in which you set the desired shutter speed and the camera decides the aperture and ISO to provide the correct exposure. You may use this setting if you are looking to give a particular sense of motion or freeze the action. Keep in mind that the camera will choose the aperture, therefore some images might have a shallow depth of field.
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Manual (M) is a full manual setting which allows you to control every aspect of an exposure. If the settings aren’t correct in Manual, the image may be under or over exposed. Manual is the best setting to use to fully understand camera settings and how they affect an image.