A practical example of mapping streams:

    ffmpeg -i <movie-with-many-tracks.mkv> -map 0:0 -map 0:5 -map 0:12 -vcodec copy -acodec copy -scodec copy "output-movie.mkv"

Use: sometimes I have a file with a lot of audio and or subtitle streams but only want one or two of each – here, 0:0 is the video, 0:5 is English audio, and 0:12 was the subtitle track I wanted. Setting the codecs to “copy” means nothing gets reencoded.