The following may be obvious but spelling it out because it's an emergency.

It is a command line. "cp" is copy. "-Rv" means recursive (copies subdirectories) and verbose (logs everything it is doing to the terminal). "*" means copy everything in the current directory (the snapshot). "~/.maildir/cur" means copy to a "cur" directory in "~/.maildir".

To invoke the command, make sure you're in the right directory (the snapshot you want to copy from), type it in, hit enter, and cross your fingers.

To get the documentation for the cp command, type in "man cp" (without the quotes).