^cu stands for "cog update" and it means to update the live version of the code. Sometimes we need to push changes that are currently breaking and NOT reload them until we've done version-control things like update libraries in production, and then reload after.
We have a whole wiki page about it: https://github.com/TsubakiBotPad/pad-cogs/wiki/Updating-Tsubaki-after-merging-a-PR