It's an open source chat/VoIP app that acts as a glossy client for the Matrix.org ecosystem for decentralised communication. You can use it to talk to anyone in Matrix or bridged into Matrix (e.g. from IRC, Slack etc), and run your own Matrix server (e.g.
http://github.com/matrix-org/synapse) so you end up owning your conversations rather than being trapped in some walled garden.