Historically, certain organizational networks were very secure and would not allow Internet access on most machines or systems, so in order to receive things like SSIN alerts, they set it up to allow one machine to poll for new alerts. The reference to "polling" is to that single machine that "polls" (searches and pulls in) for new SSIN alerts every 1 minute.
It then downloads those alerts to their SQL Server, so others could view the SSIN alerts on the network. Most customer still currently use polling unless they are new customers that already have VC7 installed. With VC7, your system connects directly to the SSIN server and gets alerts immediately, without having to dedicate a machine to download these alerts.