Map in FFmpeg can be best understood as selecting streams within a file you want to encode, keep or remove when creating an output.

-map is the command.

Streams means video, audio, subtitles, attachments and data within the file; this is rather complex on movie files where there could be multiple audio streams for different languages and the same for subtitles. Video usually has different streams for quality when it is a live stream.

Run ffmpeg -i inputfile to see all the information about the media.

You will see (depending on your input file) multiple streams, they look like:

Stream #0:0: Video: hevc .....


Stream #0:1(eng): Audio: aac ....


Stream #0:2(eng): Subtitle: ...


Stream #0:3(fre): Audio: aac ....


Stream #0:4(fre): Subtitle: aac ....


....

Examples

A simple FFmpeg map example to create an audio only and then a video only copy from the input:

ffmpeg -i inputfile.mkv -map 0:1 audio.mp3 -map 0:0 video.mp4

Because Stream #0:1 is the audio stream and #0:0 is the video stream.

-map just replaces Stream #

Making a French only audio copy with:

ffmpeg -i inputfile.mkv -map 0:3 french-audio.mp3

Keeping only the video, English audio and subtitles however encode the video (fast crf 25):

ffmpeg -i inputfile.mkv -map 0:0 -map 0:1 -map 0:2 -c:v libx264 -preset fast -crf 25 -c:a copy -c:s copy english-output.mp4

Processing what you mapped is done sequentially, -map 0:0 is the video stream and was my first map so once I had finished mapping the first action was going to be run on this video stream.

There are shortcuts too, -map 0:v selects all the video type streams as -map 0:a does for audio type streams.