Secondary sources of law can be considered as authoritative, and they help employ legal arguments. It can also be known as something that analyses and discusses primary source of law. This makes secondary source of law broad and varied both in it's purpose and nature, since each primary source can be analysed in a different method. Some secondary sources can be so authoritative that they can be relied on for court rulings.
Secondary sources of law are very helpful for legal researchers to understand any particular or general area of law. They can help you understand any law, analyse primary source, or even play a role of persuasive authority. You can also call it as a commentary on law.
Legal encyclopaedias and legal dictionaries, treatises are a few examples of secondary sources.
You can read more about sources of law following this link-
Hope this helped! Good luck with your assignment~