What is it?

source code on github

From the website :

SRS delivers rtmp/hls/http/hds live on x86/x64/arm/mips linux/osx, supports origin/edge/vhost and transcode/ingest and dvr/forward and http-api/http-callback/reload, introduces tracable session-oriented log, exports client srs-librtmp, with stream caster to push MPEGTS-over-UDP/RTSP to SRS, provides EN/CN wiki and the most simple architecture.

Basically it is a simple way to set up and get going with an RTMP server.

How to Create an SRS Server