Pretty straight forward. RPN stands for Reverse Polish Notation, and it's a different style of calculator. Watch a video, like this one (https://www.youtube.com/watch?v=7edoVNuwGvc), to learn more about it. Ask questions in the comments.
Thanks to Zro716 (https://scratch.mit.edu/discuss/topic/52783/?page=1#post-443384) for the delete key code
