Layers in a Photoshop document are like a digital "sandwich." The purpose of layers are to put the images in a different order. To make a layer appear in the foreground of the image (the top of the image) you put the desired layer before the previous layer. To make a layer disappear but not delete it you uncheck the checkbox next to it.
To organize multiple Layers and put them in a collection you can organize them in a folder. (Go to the Layers window and look at the icon left of the Layers icon. Click it and try to organize a few of your layers.)
If a photo is really dark and you want to bring out the highlights on a persons face you go to Image > Adjustments > Brightness/Contrast. Adjust the Brightness/Contrast bar.