When you post some content and make it available to your Circles, Extended Circles or Public, you decide what content you share. This is the first content filter, that gets applied to every post or every photo album you share. Each of your posts may have a different authorization list.
When you add someone in one of your circles, it means that you want to see this person publications that you are allowed to read. This is the second content filter.
So, to determine if a person R (short for “reader”) will see a content shared by a person P (short for “publisher”), P must have allowed R to read the content and R must have asked to read what P publishes. For example, you may add me to your circles even if we do not know each other. In this case, you will only be able to see my public posts, and the posts shared with my extended circles if someone in my circles also put you in his/her circles. This is as simple as it can get.