It means in MC21, the API that JRemote uses behind the scenes will allow it to ask for refreshed Smartlists. Currently the API does not allow for this, or at least not easily. There is, already, a call that will give you a new "refreshed" version of a Smartlist, but you need to know the Smartlist's ID number to ask for it.
JRemote shows the Smartlists via the normal "browse a tree" method (the same as it does to show you the results of your Media Views). So, the new command Hendrik added allows you to find out what type of view you're browsing, and get the Playlist ID if it is a Playlist type of view, so that you could then call the special "give me a refreshed playlist" command instead of the normal browse command.
This would require a new version of JRemote, but having the API there to do it is an important first step (obviously).
Hendrik, one issue here, assuming I understand it correctly is when you use "smartlist-style Views" instead of actual Smartlists. Almost all of mine are set up this way in Theater View and Standard View, at least. Wouldn't it be nicer, and easier, to have a "force refresh" option for the normal Browse/Children MCWS call?
That way, JRemote could have a "refresh this view" button for any View (whether Smartlist or not). It already has this button in the UI, doesn't it? But, you could add it via the now-standard "pull down to refresh" mechanism too, which would be cool.