Under settings then privacy such as photos why am I allowing the toggle to green and what does read or write only mean? On my iPhone 7 Plus

I just noticed that in my settings under privacy my photos and other such Apple apps are switched over to green which states read or write only and what does that mean

Settings>privacy>photos, etc,etc turn on but stated read and write only or I can choose don’t allow

I phone 7 plus

11.2.6 or whatever the latest update is my phones always updated