The first '/' is the root of the WebDAV folder tree (for copying everything) and /path/to/local/folder could be any (full or relative) path to local destination folder (can also be replaced by a dot '.' which represents current folder).
source:sourcepath and dest:destpath indicate two remotes.
rclone remotes (usually cloud accounts) has a colon after their names, that's how the program knows we're calling a remote. Right after the colon you may specify a path in the remote's filesystem (can be empty for root folder).
However for calling your local machine's filesystem (where rclone is installed) you simply specify a path without any colon.
You may want to check rclone web GUI if you're not yet comfortable with Linux terminal.