Well, I'm not an expert but you can google for a bunch of information.

The short answer is that refection is a way to discover class information at runtime.. It some situations it can allow you to approach a problem in a different way than you could without it.

Now in a partial trusted environment one of the permissons lost is the permission to interrogate other classes so code breaks. So MS gave us this nice tool and  about the time we figured it out we can't use it anymore.