I can assure you nobody will be able to give a proper answer to such a vague question without more details about what you are hoping to achieve and your background. (Is it about using ffmpeg, learning the internals, are you already familiar with how digital videos work? etc...)

These might help:

* https://ffmpeg.org/ffmpeg.html

* https://wiki.archlinux.org/title/FFmpeg

* https://ffmpegfromzerotohero.com/ (discussed 10 months ago: https://news.ycombinator.com/item?id=26370704)

Pick something you want to do then google it.

For example, I batch recorded a bunch of videos. I wanted to slice them up with ffmpeg before I brought them into the video editor. To do this, I use the command

ffmpeg.exe -ss 00:00:05.00 -i input.MP4 -t 00:00:18.00 -acodec copy -vcodec copy output.mp4

-ss is the start point -t is the offset from that start point -acodec copy copies the audio codec -i is my original video file

-vcodec copy copies the video codec