FFMpeg is a swiss army knife to process audio/video files. This an extreme oversimplification of what it can do.
In more formal words, its a robust CLI tool that can take in multiple media streams such as audio, video, livestream etc then transcode or process them and export in any number of formats.
Because its an open-source project, over time thousands of developers contributed to battle test it and make it bug-free. Overtime, may other tools started reliably depending on FFMpeg doing all the heavy lifting for them.