It's essentially a Rust rewrite of the ffmpeg CLI command, but with a modern API and good defaults + a native HTML/CSS composition engine that's been fine tuned for speed and memory efficiency. Under the hood, it still uses the libav* codecs from ffmpeg, so it can still work on a variety of formats.
Works very well with AI agents too: prompt what kind or intro/animation/graphics you need (or give it a reference), let the LLM come up with the HTML/CSS for it (Opus 5.5 is quite good at this), and let it use them on geneva.
It's way faster than the usual alternatives for putting still or animated graphics on videos (remotion, playwright etc.) because it doesn't use a browser or chromium. Everything is rendered natively, in a single pass.