The message of The Last Jedi is stated most clearly by Yoda:
“We are what they grow beyond.”
It’s about letting go of the past, but it shows that there’s a heathy and an unhealthy way to do that. Luke and Kylo do it by destroying the past that they’re unable to be rid of. Poe, Rose, and Rey learn to release it, grow from it, and protect others from making their mistakes.