The visibility of your photos is defined by two factors: privacy and safety level. Here's how it works:
You can set each photo's privacy to be seen by everyone, no one, or just your friends or family.
If someone can't see a photo in your Photostream, make sure the photo has the correct permissions for them.
If you want to share your private photos with specific people, you can create a Guest Pass. This will allow you to share your photos with anyone, and also control how long they are available.
Safety levels are used to define the kind of audience your photos are intended for.
You can have safe, moderate, or restricted content. Moderate and restricted content are only visible to members who modify their SafeSearch preferences to see that level of content.