I briefly explain what the play out server / media server actually does. In order to make the streaming size more manageable, the stream first is encoded. You can compare this to zipping files. You squeeze gigabytes down to megabytes and then package it into a protocol like RTMP to stream it over the internet.
Connect your video feed to a media server
To connect your video feed in OBS to a play out service, you need a digital key. The key is a set of characters you can copy and then paste into a field in OBS. Once this connection is established, you can go live.
Once you are ready to show something live to an audience on the web, you need to stream your feed to a distribution hub or media player. In technical terms: Ingest the feed.
Play out online video
These services for distribution allow you to scale up to a bigger audience. The most known (free) services at the moment are YouTube, Facebook Live, Vimeo Livestream and Twitch. They will repackage the stream to a protocol like HLS or MPEG Dash and sent the stream to different compatible devices.
When you want to go live with video, consider this: Multistream to 30+ Platforms Simultaneously.
Here you find my popular article that serves as a primer on video streaming.