The preimage is the result of merging the old and new files, I get that. The postimage is the way the conflict was resolved, I get that.
But what is thisimage?
As explained by bitbucket.org:
“thisimage” is a bookkeeping mechanism rerere uses during a resolution. It’s possible an unlucky file can have many conflicts, which will all be within this directory. “thisimage” is the one most recently being worked through.
Now, when rerere sees a conflict for file.txt, it looks in the corresponding rr-cache dir, find the “preimage” that matches “thisimage” and changes the file in the working directory to match “postimage.”
https://bitbucket.org/blog/resolving-conflicts-with-git-rerere