"Na nach nachma nachman meuman" is a phrase that some people say or sing to show their love and admiration for a religious leader named Rabbi Nachman of Breslov.
It's like when you really like a superhero, and you might sing a song about how awesome they are. Some people who follow Rabbi Nachman think he was really special, so they sing this phrase to show how much they like him.
Each word in the phrase has a special meaning to these people- "Na" means "please," "Nach" is a nickname for Rabbi Nachman, "ma" means "what," "Nachman" is his name again, and "meuman" means "from Uman," which is a city where Rabbi Nachman is buried.
So when they sing "Na nach nachma nachman meuman," it's like saying "Please Nachman, what Nachman? From Uman Nachman!" It's just a way for them to express their love and devotion for this religious leader.