Here is how to extract part of a video.

# extracts 5 minutes
ffmpeg -i input.mkv -ss 00:20:00 -c copy -to 00:25:00 output.mkv 
# extracts 25 minutes, if the video is long enough
ffmpeg -i input.mkv -ss 00:20:00 -c copy -t 00:25:00 output2.mkv

Probably from here with tags explained and everything. Seems that the correct name is ‘seeking’.