It is a replacement for Blizzard's Floating Combat Text and Damage output.
Description:MSBT is designed to be an extremely lightweight, efficient, and highly configurable mod that makes it easier to see combat information by scrolling the information on the screen in separate, dynamically creatable scroll areas. Version 5.7.138 corrects issues with several triggers like Kill Shot and Hammer of Wrath.Frequently Asked Questions:Before posting a question, please see the FAQ list at MSBT's portal site. If you are having issues with the fonts 'resetting' on initial game load, update to Version 5.7.137.
Version 5.7.134 is updated to work with Warlord of Draenor Patch 6.0.2 and fixes the font preview issues.