This function uses ffmpeg to trim the beginning and end off a supplied media stream.

From the ffmpeg documentation (available at https://ffmpeg.org/ffmpeg.html):

"ffmpeg is a universal media converter. It can read a wide variety
of inputs - including live grabbing/recording devices - filter,
and transcode them into a plethora of output formats.

ffmpeg reads from an arbitrary number of input "files" (which can
be regular files, pipes, network streams, grabbing devices,
etc.), specified by the -i option, and writes to an arbitrary
number of output "files", which are specified by a plain output
url. Anything found on the command line which cannot be
interpreted as an option is considered to be an output url."

In this case, the input url is supplied as \$1, and the output url as \$4. The -ss option in this context tells ffmpeg to discard all input until the position supplied as \$2, and the -t option tells it to stop writing the output when the duration reaches \$3. Both \$2 and \$3 are time durations, which can be specified in various ways (as discussed here: https://ffmpeg.org/ffmpeg-utils.html#time-duration-syntax).

-vcodec copy and -acodec copy tell ffmpeg to omit the decoding and encoding step for the audio and video streams, so it does only demuxing (demultiplexing) and muxing (multiplexing) steps. The documentation notes: "Since there is no decoding or encoding, it is very fast and there is no quality loss. However, it might not work in some cases because of many factors."
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