Start up disk
"It's telling you that because your.. (don't be shocked) start up disk is almost full. Which is a Bad Thing™. OS X really needs to have at least 1.5GB of free disk space so it has at least a gigabyte to swap (write RAM to disk), and an extra 512 megabytes for some elbow room.
The fix? Delete stuff you don't need. Start with your home directory. Alternately, get an external hard drive and copy stuff to it from your home directory. "