ffmpeg is a very versatile and useful tool with the capability to compose and manipulate multimedia in all sort of complicated ways… Emphasis on complicated. However it is still possible to use ffmpeg for simpler things without having to learn all of its syntax and arguments. And one of the simplest things you could use ffmpeg for is to simply convert between various media formats.
ffmpeg is rather smart. Very smart, in fact. If you want to convert between formats, it can usually detect what you want to do just by the file extension alone. For instance, if you want to convert an .mp3 file into an .ogg file, you simply specify the input file (preceded with a -i), and then a new output filename with the file extension for the format you'd want to convert it into.
ffmpeg -i music.mp3 music.ogg
If you specify the same file extension then it will simply reencode the file. Depending on how the original file was produced, it could significantly decrease the filesize even with just the default encoder settings ffmpeg provides. When reencoding videos I take on my phone this way, they usually almost halve their filesize.
ffmpeg -i music.mp4 music.mp4
Even though ffmpeg is mostly associated with video and audio, it can also work on images too. Say, you've downloaded an image from somewhere on the internet, but it is a WEBP and you'd rather want it in PNG. Converting it from the terminal is as simple as:
ffmpeg -i image.webp image.png
If you've downloaded some music video using yt-dlp or the like, but didn't specify to extract the audio (which can be done with -x, by the way), you can do this with ffmpeg. However, if you try to convert into an Ogg Vorbis audio file, you may (or may not) be surprised to see an Ogg Theora video file being produced instead.
As .ogg is merely a container, it supports more than the Vorbis audio format, including the Theora video format. So when you specify a video file, ffmpeg expects to put both the audio and video in the output file, even though most of the time you only want audio in an .ogg file. To specify to ffmpeg that you only want the audio in Vorbis format to be put in the .ogg file, you'd want to add the -vn argument after the input argument:
ffmpeg -i music.mp4 -vn music.ogg
Depending on the build of ffmpeg you have at your disposal, it might have been compiled with libopenmpt support allowing you to render tracker music into a more traditional audio format:
ffmpeg -i pod.s3m pod.ogg
However, if your ffmpeg build for some reason does not include libopenmpt (/!\ or it uses libmodplug which has terrible playback quality /!\), openmpt123 also has the ability to render tracker music into a traditional audio format. The formats supported are unfortunately limited, only FLAC and WAV are supported… But of course, you could render to FLAC and then convert to a more well compressed format like OGG Vorbis, using ffmpeg!
openmpt123 --output-type flac --render pod.s3m
ffmpeg -i pod.s3m.flac pod.ogg
I hope these examples show ffmpeg's usefulness even when doing something with it that doesn't require learning a lot of its arguments and syntax.