In previous versions of Microsoft Office you had to choose a security level, for example, Medium, which would then apply for the whole program — so every document you opened was opened under the same conditions. In addition, security decisions about documents had to be made before you could see their contents, so many people just clicked straight through the security warnings, accepting any risks without thinking about it, in their eagerness to get into their documents.
In the 2007 Office system, documents automatically open with potential security risks disabled, so you can see what the document is about and then decide if you want to enable any macros or other features in it. A great thing about the file opening with everything disabled is that you can read through the contents of the file first. And most importantly, by using the Message Bar, you can choose what to do in any given file each and every time you open it.