First a warning: Microsoft Defender is rated very low. See the pic with the red bars in http://www.zdnet.com/article/antivirus-on-windows-88-1-compared/. If you want it free, avast, avira or avg are far better choices.
Most antivirus programs now come with a module that in some way check the links shown by a search engine or the links that you click against their database before you actually load them to minimise the risk of going to unsafe sites. If they don't have, they still should check the stuff coming from that site when you really load it. I thnk that the Norton message relates to some pro-active feature like that.
People here quite often use WOT as their reputability checker.
My Norton antivirus program is about to expire and I was going to switch to Microsoft Antivirus, but will it have issues with Google's Chrome Browser instead of Internet Explorer as a browser?
Have been using Norton with no problems until recently when I got a message to upgrade it. Doing so got me blue screen. Now I get a Norton message that my Chrome Browser is not protected and to install Norton Chrome extensions. So it seems Norton has a problem with Chrome also.