WordPress MU version 2.6.1 has just been released. This is a sync of the WordPress maintenance release that saw the light of day a few weeks ago.
This is a required upgrade as it fixes a number of critical bugs, particularly in deleting users and blogs. Upgrading is as easy as copying over the files of your current install.
For a more comprehensive list of changes, check the timeline, but in short, yes you do need to upgrade!
WordPress MU is a multi blog version of WordPress that runs many sites such as Linux.ie Blogs and WordPress.com. If you run the single blog version of WordPress you can probably ignore this message.