This notation is frequently used in technical forums or software archives to identify a specific, legitimate installer file for older Apple computers. If you are trying to install this, I can help you find: for Mavericks. Instructions on how to create a bootable USB drive. Known issues with this specific 10.9.3 release.
This indicates the third major update to Mavericks, which was released in May 2014, known for improving 4K display support and syncing capabilities [1].
This is OS X 10.9, released by Apple in 2013 [1].