This means that whatever symbols you're looking at on Google Finance will be updated without delay, and the changes can be seen both on the page and at the top of the tab it's open in on your browser. You can also get it in widget form, either in iGoogle or on your phone with Google's mobile-alerts service.
Earlier this month, Google, The Wall Street Journal, and CNBC partnered with Nasdaq to get real-time quotes from that composite index. Like NYSE, quotes from that index were delayed up to 15 minutes, rendering the service less of an asset to time-sensitive trades.
It should be noted that most serious day traders use proprietary subscription-based brokerage and charting tools that integrate with buying and selling services. Many would likely consider Google's current offerings not quite up to snuff in comparison, although the addition of real-time quotes may make it easier for consumers to view rapid fluctuations on potentially volatile stocks that the slower systems would not have illustrated.