You can think of Stream Feeds similarly to a Stack. That is to say, a Feed is a Last In First Out situation (LIFO). Similarly to my Instagram feed, the most recent picture I posted will be the first returned when I fetch the data by loading the app.
By default, Activities will be returned from the Stream API in Reverse Chronological order, as described above. However, this can be changed with the addition of a Ranking Algorithm. A ranking algorithm allows you to choose the order of activities returned by scoring each one against an algorithm. Here are some ideas for ranking algorithms