Holochain is a data integrity layer for distributed applications. Basically a way to have both the applications and their required data storage run on all of the users machines instead of centralizing it on a server.
It is a framework that is made to be agent-centric. That is to say, that all applications built on Holochain will enable the agent (person, device or collective) to communicate with other agents through a bunch of applications it has installed.
This means that an agent stores all of the data connected to the agent and allows this data to be accessed by applications of the agents choice. Agents also hold a share of other agents data, one piece of the collective dataset through the use of Distributed Hash Tables.