Although we may casually share clips of content with friends in our online world, this doesn't apply to serious academic inquiry. 
Giving credit at every opportunity to the work of others in academic writing is essential to show how we develop arguments and viewpoints. Because we build on the work of others in order to form new knowledge, we must cite the work that came before us to help readers understand how we reached our conclusions.
Cartoon via grammarly
Not every statement in your writing needs to be cited. Every discipline has a generally accepted definition of what is considered "common knowledge" in that discipline, and those statements need not be attached to a citation.
This handy flowchart can help you decide:
Why your teachers insist on accurate citations
University of Toronto librarians asked instructors why they insisted on accuracy in listing sources and using in-text citations.
Content from the University of Toronto Library Guides.