This is an incredibly small and simple UI mod which exposes a new feature added in WoW patch 1.10.
With this mod enabled, you can use your cursor keys to edit the chat lines and commands you type in the editbox.
Normally, to move the cursor left and right you'd have needed to hold ALT while pressing the left or right cursor keys, which is awkward -- and to go forwards and backwards in the command history you'd have needed ALT+Up and ALT+Down.
Not anymore. When you're editing the line, the cursor keys just work, without holding ALT. Much more comfortable (at least, I think so).
Note: This mod may not necessarily be suitable for those strange souls who actually use their cursor keys for character movement, but for those of us who use the more conventional WASD or ESDF, it's absolutely fine.