The above command would copy the file 'myfile.txt' on the C: drive to the D: drive. You can use the copy command from the command line to copy files. For general information about using the command line (cmd), see: How to use the Windows command line (DOS).
The following examples use the Windows command prompt to copy files from one drive to another. For more information, see: How to copy files.
There are other ways to copy files, and you can use whichever you prefer.