It's Z, the traditional undo key in Sokoban-likes. More specifically, the key where Z would physically be on a Qwerty keyboard, regardless of what keyboard layout you have actually selected. Same goes for [R]eset.

Was almost surprised that a little web game gets this right when many native apps don't. I just checked the last three Sokoban-likes I played (Stephen's Sausage Roll, Baba Is You, A Good Snowman Is Hard To Build) and they all follow the currently active layout. Which admittedly is way better than the really annoying games that memorise the layout that's selected when you launch them and have to be completely restarted if it wasn't the one you wanted.