It's a Message Content Macro... you use it in a message template to include the contents of your Windows clipboard in a message at a given location. This is one of those functions that I can imagine being rarely used, as the contents of the Windows clipboard changes constantly... so unless you really want to send your correspondents a copy of the last thing you copied it's not that useful, imho!
Anyhow, if you want to know how it works.. this is how... firstly go to the Account menu ยป Properties and scroll down to the Templates section. Choose which template you want the Clipboard macro to work with and then decide where in the message you want the copied text to appear... then add this %CLIPBOARD to the template. Click OK to save the template and close account properties.
Then if you have a block of text in any program you want to add to a message, you would first copy the text to the clipboard. (Highlight and press Ctrl+C). Then open the new message or create a reply and you'll see the contents of your clipboard already added to the message.
As I said before, it's not a very useful Macro in my opinion... but that's how it works. :-)