The goal of the "Observing" section of this website is to help us learn to observe.
"Phenomena" means "appearances." (Singular: "phenomenon.")
"Basic celestial phenomena" refer simply to how the sky looks and how it appears to change over time. The stars and planets are constantly moving from night to night, and changing from season to season. Other changes become visible only over several centuries.
If we can observe it, then it's a "basic celestial phenomenon," and hopefully the pages here will help us attend more carefully, with greater awareness, and better understanding of what we are seeing.