In your first assignment, you played around with the Aperture and ISO settings with you photography. You may have noticed, that while using certain settings, that the Shutter Speed (the clicking noise) had different delays to them. There’s a reason for this…

The Exposure Triangle (or Photography Triangle) is the balance between Shutter Speed, Aperture, and ISO – and how it can affect the results of your final image. Understanding this balance can be a bit tricky at first, but practice and experimenting will help you to learn to use these concepts.

Here’s some notes about this concept: Triangle

Also, here are some additional resources you may find helpful:
Exposure Explained (Video)
Technical Exposure Explanation (Video)
What is Exposure (Article)