What is the ASS format?

ASS files are written in Aegisub and stored as plain text. Besides the timing and text they define styles: font, size, colour, outline, where on the screen a line appears, and effects such as karaoke highlighting or moving signs. That is why fan subtitles for anime are almost always ASS.

Only some players draw all of it: VLC, mpv and MPC-HC do, many TVs, phones and video editors do not. Converting to SRT or VTT keeps the text and timing so the subtitles work anywhere, but most of the styling is lost.