P mode is used for people who aren't expirenced in taking photos
Manual mode your able to chose your own settings & can even lock them down which can help expose the picture more clearly
The more the exposure is increased in a photo the more light it will have
ISO is known as the activity of the cameras sensor, which can be between 100-6400
The more ISO added to the picture the brighter it is, the less you use causes a darker picture
The shutter speed can always be changed by going to the shutter priority mode