RTSP stands for Real Time Streaming Protocol. Basically, it is a network protocol designed to transmit audio and video between a server and a client. Originally designed for use in entertainment and communication systems, the protocol supports functions such as play, pause, skip, etc.
RTSP has many uses, such as movie streaming and CCTV cameras. Most professional security cameras these days use this protocol as the standard for live audio and video streaming.
RTSP supports many audio and video codecs including mjpeg, mpeg4, h.264, h265, mp3, etc.
For a detailed explanation, see this link from wowza: https://www.wowza.com/blog/rtsp-the-real-time-streaming-protocol-explained
