Plex is a global streaming media service and a client–server media player platform, made by Plex, Inc. The Plex Media Server organizes video, audio, and photos from a user's collections and from online services and streams it to the players. The official clients and unofficial third-party clients run on mobile devices, smart TVs, streaming boxes, and in web apps.
Plex: https://support.plex.tv/articles/200288286-what-is-plex/