Common knowledge is information that can generally be assumed to be known by the average person in a given community or discipline. However, it can be difficult to determine what information is considered common knowledge.
Your instructors and librarians recommend that you THINK CAREFULLY before deciding not to cite something. Ask yourself: Is this something I knew BEFORE doing research on my topic? Would a student in my class know the information I am claiming is common knowledge?
The Purdue Online Writing Lab (2019) offers the following guideline: "Generally, common knowledge is information that someone finds undocumented in at least five credible sources" (What is common knowledge section).
When in doubt, it is better to cite than to risk being accused of plagiarism.