Synapse is an open-source Matrix homeserver written and maintained by the Matrix.org Foundation. We began rapid development in 2014, reaching v1.0.0 in 2019. Development on Synapse and the Matrix protocol itself continues in earnest today.
Briefly, Matrix is an open standard for communications on the internet, supporting federation, encryption and VoIP. Matrix.org has more to say about the goals of the Matrix project, and the formal specification describes the technical details.