According to release details, the Office for Mac 2011 update contains changes to add functionality and improve security and stability in the program.
Additionally, the update includes fixes for vulnerabilities which would allow an attacker to overwriter the contents of a computer's memory with malicious code. That's a pretty big vulnerability, so any users of Office for Mac 2011 should probably get right on this download.
From Microsoft's support website, the update includes:
This update fixes vulnerabilities in Office 2011 that an attacker can use to overwrite the contents of your computer's memory with malicious code. For more information, see the security bulletin that was listed earlier in this document.
This update fixes an issue that causes Office 2011 applications to close unexpectedly when you open or use Office 2011 applications.
For a full list of improvements see the release details here.
This update enables the authoring of Alt Text for objects, such as shapes, pictures, tables, charts, SmartArt, and movies in Office 2011 applications.
To obtain the update, head to the Microsoft Download Center or use the Microsoft AutoUpdate feature on your Mac.