In United States copyright law, “fair use” allows creators to incorporate copyrighted materials into their own work—without obtaining the permission of the copyright owner—when certain conditions are met.
Fair use recognizes that while the primary purpose of copyright laws is to encourage artistic and cultural innovation, rigid application of copyrights would actually stifle that creativity. Society benefits when creators have the freedom to critique and comment on the works of their peers, or to remix and reuse artistic material in new and inventive ways.
If you’re interested in learning more about fair use and copyright laws, check out the U.S. government's official word on the subject. Stanford University also offers a pretty comprehensive overview that’s worth a look.