Please refer to this post on how and why to install HomeBrew (a command-line package manager).
You can install ffmpeg quite easily thanks to homebrew by opening a terminal and run:
brew install ffmpeg
Anyway a few components are missing from that package, like ffplay which is a great too to preview the scripts before write to disk.
please check this post on how to get it
This because you need to compile ffmpeg from source. So it’s up to you, get it as it is without going through Apple crap or accept their conditions and get the full package.
if you do so you can then run:
brew install ffmpeg --with-sdl2
if you need to remove ffmpeg run:
brew uninstall ffmpeg
or if you have broken dependencies:
brew uninstall --ignore-dependencies ffmpeg
I’ve got a few guides here on my blog on how to use ffmpeg, check them out :)