If you record your videos with OBS, you can record them in the format of mp4, which is actually container format, which includes video, audio (and other stuff like subtitles for example) in a container file. With ffmpeg, you are able to extract and work with those streams in the containers. That means, you can extract and edit streams directly from and inside the container without rendering the whole video file, which saves a huge ammount of time and resources.
Go to ffmpeg.org and download the latest build for your system. If you are using linux, you can also install it by your package manager of course. If you decided to download the binaries, place them somewhere you want (on Windows C:/Program Files/ffmpeg/ for example) and define the location of the "bin" folder in the ffmpeg folder in your path variables.