They say that it protects your computer against viruses, spyware and other kinds of malicious software, all in real-time. What I think that is bad, simply because it is made by Microsoft, is that it may fail regularly in different situations (fake alarms, issues when running, weak detection and stuff). But this is not something proven and is based on my past experience with Microsoft's products.
Those who tested said that it detected and resolved several threats better than NOD32, and even used less resources, but I doubt that this is something relevant (as I haven't seen any "serious" tests). There are comments on blogs where this product was reviewed in which the people told that it is a very bad protection program, because it wasn't able to detect important trojans/viruses so my doubts come also from here.
You can download and try Microsoft Security Essentials from http://www.microsoft.com/Security_essentials/ .