I’m not sure what you’re asking for. That’s the command. Unless you meant an explanation?

The basic command is convert filename.cr2 filename.jpg.

That parallel command runs the convert command on all of the .cr2 files in the current directory, running a bunch of them simultaneously. {} is replaced with the name of a file, and {.} is replaced with the filename without the extension.

https://www.gnu.org/software/parallel/parallel_examples.html

If you didn’t want to use parallel and are okay with it slowly converting one file at a time, you can just use a for loop:

for file in *.cr2 ; do
  convert $file ${file%.cr2}.jpg
done

That one uses some Bash variable magic to remove the .cr2 and add .jpg to the file name of the output file.

convert is smart enough that you can just give it an output name ending in .jpg and it knows it should convert the input file to JPEG.