This error means that there was a problem when Front Page attempted to communicate with the server.  Possible causes are:

Network congestion through your ISP or static on the phone line you're connecting through may have caused a communication error.  The first thing to try is canceling out of the error and trying to publish again.  Often times canceling and re-trying will clear the problem

Not being connected to the internet or a problem with your internet connection.  Many services like AOL have a timeout feature that automatically logs you off after a certain amount of time.  Check to make sure that you are still logged on.  Even if you have an 'always on' connection like DSL or Cable there could be an issue with the connection.  Try going to your web browser and typing in a well known website you've never visited before like etoys.com or amazon.com to verify that your connection is working.  

A slow internet connection.  A slow connection may cause a timeout error on the server.  Essentially your computer isn't communicating enough so the server assumes it's finished.  Keep in mind that some 'high-speed' connections have super fast downloads but very slow uploads (some as slow as 14.4) so it may take a few tries to pass all of your files to the server.

If you've used FTP to upload files to a FrontPage site you have likely corrupted the Front Page extensions.