What don't you get? How a negative index works has already been explained. I would think you understand how basic arithmetic and addition/subtraction works so if you substitute values and do the math,
you should be able to figure out what index values are used. I imagine you know how for loops work by now. And the end='' part just suppresses a line feed. Part of learning how to program is learning how to break down code into its component parts and then reassembling it again to understand how they all work together.
Why not give it another shot yourself and try to explain what your understanding is? That way we can see what exactly you're missing.