ffmpeg has been described as the Swiss army knife of multimedia. It incorporates hundreds of video, audio, and image decoders and encoders, muxers and demuxers, filters and devices. It provides a CLI and a set of libraries for working with its tools, and is the core component of many video and audio players as a result (including my preferred multimedia player, mpv). If you want to do almost anything with multimedia files — re-encode them, re-mux them, live stream it, whatever — ffmpeg can handle it with ease.