The current version of the operating system for Macintosh computers (OS X, not to be confused with iOS, the operating system for handheld devices) is code named "Mountain Lion", but its official number is 10.8. There have been several updates to 10.8 since it first came out, and these have gotten sub-version numbers tacked on. Up until today, you were up to date if you had sub-version 10.8.4, but Apple's taking one last kick at the can by releasing 10.8.5 in hopes that everybody will have it installed and in place, all neat and clean, when the new baby (Mavericks, 10.9) comes home later this year.
In case you don't get Apple's announcement directly, go to your menu, select "Software Update", and click on the "Update" button for the software update that appears in the App Store window. The download will take about 5-10 minutes, depending on your connection speed. Then follow the on-screen instructions. A restart will be required.
We encourage everyone to stay up to date on the latest system software that your hardware is able to handle.