What is RTMP?
RTMP is the Real-Time Messaging Protocol, the ingest standard streaming software uses to push live audio and video from your computer to a platform's servers.
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You give your encoder two values: a server URL beginning rtmp:// or rtmps:// and a stream key that identifies your channel. The protocol carries the stream to the platform, which then transcodes it into the formats viewers actually receive. Delivery to the viewer is handled by other protocols, so RTMP is the upload half only.
Knowing this is what lets you stream anywhere a custom RTMP destination is accepted, including platforms with no dedicated integration in your software. It is also the first thing to check when a stream will not start: a wrong or expired stream key and a blocked outbound port account for most failures.
Treat the stream key like a password, since anyone holding it can broadcast to your channel, and reset it if it appears on screen during a stream. Prefer the rtmps:// endpoint where a platform offers one, as it encrypts the connection, and note that some newer services now ask for SRT or WHIP instead.
Where RTMP shows up
You will meet RTMP most often while comparing live streaming software - StreamElements, Evmux, Riverside, and StreamYard all deal with it, though not in the same way or at the same price.
Related terms: Bitrate and Multistreaming.