A Shared Action is an Advanced Action (standard or conditional) that can be reused multiple times in a single project or shared across multiple projects.
Shared Actions make sharing actions across projects easy. But there are other ways to share actions across projects. Dr. Pooja Jaisingh has written a great blog post about this that you can find at: https://elearning.adobe.com/2013/12/the-most-awaited-enhancements-in-advanced-actions-and-variables-are-here/.