We call the images on the foreground layers "objects". For example, if you import a photo of a shoe, the shoe is an object on the foreground layer.
Photoroom tries to name each layer based on the type of object that appears on it.
Each layer has settings that you can use to edit the object on that layer. This is how you change the color of objects and apply effects such as shadows and reflections.
To learn about editing objects, see: