A Holocron is an information storage device which was used by force users of both the Jedi and Sith Orders. They often contained ancient knowledge, instructional lessons and even maps to guide those who opened it.
Most Holocrons were able to deliver their contents through a guardian, who could consciously decide whether or not to share the information inside. This guardian often appeared as the person who crafted the holocron and was able to both restrict and grant access to different information inside.
To open a Holocron, a person must attune to it using one of the three aspects of the force. A Jedi holocron could only be opened using the lightside, a Sith holocron could only be opened by tapping into the dark side and Grey Holocrons could be opened using either. Interestingly though, the knowledge granted by Grey holocrons depended on which side of the force it was opened with. If a person chose to use the dark side to open it, they would be delivered far, far different information to someone who used the light.