The first rule of copyright
Creators should only upload videos that they have made or that they're authorized to use. That means they should not upload videos they didn't make, or use content in their videos that someone else owns the copyright to, such as music tracks, snippets of copyrighted programs, or videos made by other users, without necessary authorizations.
What is Fair Use?
Fair Use is a U.S. law that allows the reuse of copyright-protected material under certain circumstances without getting permission from the copyright owner. However, Fair Use is determined on a case by case basis, and different countries have different rules about when it’s okay to use material without the copyright owner’s permission. In the U.S., works of commentary, criticism, research, teaching, or news reporting might be considered fair use, but it can depend on the situation.