Copyright is a law that stops you from using other people's work and claiming it as your own. This work could include music, videos, images, and even ideas.
To use copyrighted work you must get permission, mention the person's name if you use their work, and sometimes pay a fee.
There are a few exceptions when it comes to copyright. This is where fair use comes into play. If you are wanting to use someone's work for educational purposes such as research or a news report, fair use keeps you from getting in trouble. Beside this I have linked a website to help you better understand fair use.