Common Practices offer particular practices and approaches that may be helpful in your own practice of Scrum. None of them are required elements of Scrum, but they are all things of which any would-be Scrum practitioner should be aware. Some Common Practices are very broadly applicable. Others may apply only in special circumstances. As someone using Scrum, you should be aware of them, and should carefully consider the pros and cons of using these practices.
Note: "Common" does not mean "Generally Recommended". These are practices that many people recommend, but often many others do not. Often a topic here will be controversial and there may be articles presented for and against the idea. You are invited to reflect and consider all sides as you inspect and adapt your own Scrum process.
Welcome to the broad world of Scrum understanding and practice!