What is MPlayer?
MPlayer [mplayerhq.hu] is a video player that is used to create video players. More specifically, MPlayer is a free and open source project that implements the essential features a video player needs (such as reproducing video). Several video players, specially for Linux, call themselves "a frontend for MPlayer," which means that the program that reproduces the video is MPlayer, and that specific Linux video player is just the graphical user interface to control MPlayer.
It seems MPlayer does provide its own GUI, although not by default anymore, but since this isn't the focus of the project, it's a very simple GUI compared to what you would expect from a fully featured video player.
Examples of Video Players based on MPlayer
Quotes
SMPlayer is a graphical user interface (GUI) for the award-winning MPlayer, which is capable of playing almost all known video and audio formats.
https://www.smplayer.info/ (accessed 2025-04-20)
mpv is a media player based on MPlayer and mplayer2
https://mpv.io/manual/master/ (accessed 2025-04-20)
In this case, "based" means they are a fork of it.
mpv is based on MPlayer's code. At first, there was the mplayer2 project, which forked away from MPlayer roughly in 2008, and went different ways from it. (It was renamed to mplayer2 only a few years later, causing general confusion.) In 2012, some newer developers, who joined mplayer2 development around 2011, forked mplayer2 again. This second fork was then renamed to mpv.
The reason for this fork was disagreement over the direction of development. While MPlayer focuses on maintenance and not breaking old code and features, mpv wanted to go into the other direction, modernization. This included merciless removal of code/features that were not considered worth maintaining due to being obscure and unused or rewriting existing code to clean it up and to reduce bugs. Reducing maintenance burden by replacing MPlayer code with FFmpeg mechanisms was also a priority. mplayer2 was stuck between those positions - first going for mpv's direction, but then remaining stagnant with slow development. The mplayer2 project soon went inactive after the mpv fork, and is completely gone today.
https://github.com/mpv-player/mpv/wiki/FAQ (accessed 2025-04-20)
Celluloid (formerly GNOME MPV) is a simple GTK+ frontend for mpv
https://celluloid-player.github.io/ (accessed 2025-04-20)