Can I use
ffmpeg on my iPhone and edit videos in my camera roll with it?
If your device is jailbroken, you can install the ffmpeg package:
This package is open source on GitHub nin9tyfour/FFmpeg-iOS-build-script which you can use to build a more recent version and manually install it yourself if the version of ffmpeg provided by this package is insufficient for your needs.
You can use the package from the command line using a Terminal.
A terminal emulator package, installable with Cydia.
NewTerm is a continuation of the Mobile Terminal project, a versatile terminal emulator for iOS.
You can also use an SSH app from the App Store, however this requires you to change your SSH port from 22 since iOS prevents localhost access to port 22, so suggestions for apps and instructions are out of scope for this question.
In a word NO.
FFMPEG is a command line video converter and iOS has no command line. There are plenty of programs that can edit video though, not the least of which is Apple's own iMovie.
A simple search on the iTunes store (the only place you can install software from) for "edit video" will turn up a lot of apps.
(Yes I know all about jailbreaking and what is possible, command lines and alternative applications, etc. But we do not discuss or support people with Jailbreaking issues, there are other places for that)
ffmpeg is both a command line utility and a software library allowing modification of various media files. As a command line utility you won't find
ffmpeg in the App Store, as there's no command line support in iOS.
However parts of
ffmpeg are also used as a software library (usually in the form of
libavcodec) in a wide range of applications, some on iOS as well. For example VLC on iOS uses it to decode and display media files. My searches did not show up unfortunately any good apps that use either ffmpeg or libav for encoding & editing purposes, but it's likely a few of the video editor apps you'll find will use these software packages as their backend.