Open licenses allow content creators to specify which uses they permit for their work without permission and under what conditions. The most popular open licenses are Creative Commons licenses.
Open licenses do not replace or annul copyright. Openly licensed content still belongs to its creator, but specific uses are permitted to the public under each license type. Some are very permissive and some are more restrictive.
When you use CC-licensed content, time spent searching for a copyright owner to ask permission to reuse, remix, or share “All Rights Reserved” content is channeled to research, create new works, or teaching.
Different CC-licenses permit and/or limit specific uses. Here's an infographic to help explain what is permitted and required by each license (in context with public domain and standard "all rights reserved"). Licenses are organized top to bottom from most open to least: