ffmpeg -i source_video.mp4 -t 50 -c copy output_video.mp4
This cuts the first 50 seconds of the video named source_video.mp4 and outputs them without any re-encoding to output_video.mp4
Another example for audio files:
ffmpeg -i input_file.mp3 -ss 00:00:05 -t 00:00:21 -c copy output_file.mp3
This cuts the audio file from the 5th second to the 21st second and places it in a file named output_file.mp3, no re-encoding done whatsoever.