A simple background in photography is a type of background that doesn't drown out the main focus of the photo. For instance, if I need to take a picture of a red apple, I'm not going to use a packed stadium as the background. It's too wild and drowns out the red apple. Hope this helped!
If someone says to use a simple background in photography it means to have a nice background that compliments the main focus of the image you are trying to take. It should not take anything away from the main focal point.
