Who shouldn't know it yet: FFmpeg provides numerous codecs and tools for processing audio and video, which can be used independently (standalone) or as functions of other programs and applications. The special thing about FFmpeg - besides its open development with visible source code - is the wide range of supported codecs. That's why FFmpeg is now used in an astonishing number of (and even many commercial) applications.
However, the use of FFmpeg also has its "price". Since the developers of many codecs do not want to or cannot accept patents and possibly due license fees, they only provide the source code of FFmpeg, which you have to compile yourself to a finished program. This procedure is one hundred percent legal in most countries, but compiling programs is not common to almost any user (and not always trivial even for experienced developers).
If you want to make it easy for yourself, you can find compiled versions on the net, whose (at least commercial) use may not be as legal anymore. However, even if a supported codec is not used legally, tracking is very unlikely, partly because the license thicket for codecs has recently become virtually impenetrable for end users. Even for specialist lawyers.
Unless you run a commercial transcoding service in the cloud with FFmpeg, the use for your own video production is hardly more problematic than the use of commercial codecs. In the new version of FFmpeg 4.0 some new codecs have been added. For example, the relatively new AV1 codec can now be tested and used by interested users. In the audio sector, the aptX and aptX HD codec is now available, which is primarily used for high-quality audio transmission via Bluetooth.
The new version FFmpeg 4.0 now supports Nvidia's proprietary interface for video encoding and decoding (NVENC/NVDEC). Hardware support for special AMD and Intel hardware has also been improved: Intel's QuickSync and VAAPI interfaces now also accelerate VP8 decoding, MJPEG encoding and encoding, and overlay filters. With supported AMD hardware, accelerated AVC/H.264 and HEVC/H.265 encoding is now available.
Recently, metadata of common MPEG2/AVC/HEVC bitstreams can be changed via filters without re-encoding. Previously, for example, the entire stream had to be recoded to change the aspect ratio.
With the new version, support for Windows XP has been removed. However, this does not happen intentionally, but is due to the dependency on third-party libraries, which no longer offer Windows XP support. If you still need it, you can create a functionally limited Windows XP version thanks to the open code. However, the necessary know-how of project configuration should only justify the effort in very few special cases.
Who shouldn't know it yet: FFmpeg provides numerous codecs and tools for processing audio and video, which can be used independently (standalone) or as functions of other programs and applications. The special thing about FFmpeg - besides its open development with visible source code - is the wide range of supported codecs. That's why FFmpeg is now used in an astonishing number of (and even many commercial) applications. 
