That creates a macro.
You have "use preview list to build macros" turned on, and when you edit filenames in the preview list, that turns on "macro operation" and, in this case, set the macro to L0-13, i.e. remove the first 14 characters from the left (or something like that; I never edit those codes by hand as you rarely need to).
When you click Apply the first time, it removes the first 14 characters (or whatever the macro does). It then shows a preview of what will happen if you were to do that again.
Click Reset to clear the macro if you want to do additional, different operations on the files. Or click Cancel if you're finished.