ffmpegfs is a free and open source FUSE-based read-only transcoding filesystem which converts audio and video formats on the fly when opened and read. It supports many formats, including MP4, WebM, OGG, MP3, OPUS, MOV, ProRes and WAV, among others. This is useful in case you have many files in your media collection that can’t be directly played by some hardware or software (e.g. DaVinci Resolve, which has limited codecs support in the free Linux version) – instead of transcoding the whole media collection you could use ffmpegfs to transcode the files on the fly, when they are accessed / played.