The most prominent tattoo on Skeen’s body is on his chest. There’s a symbol at the top and, just below it, are two letters in Aurabesh: K and H. Cassian calls this out as “Krayt Head.” Below that is a barcode and a string of numbers, marking him as a prisoner of something or another. The other symbol that they speak of, on Skeen’s arm, is something Cassian recognizes as “By the Hand.”
Now, these two phrases haven’t had any use in “Star Wars” in this configuration before, but using context clues from their conversation and the real world inspiration of such tattoos, we can come to a pretty solid understanding of what they mean.
As they talk around their mistrust about each other’s backgrounds, they find common ground through the tattoos that the others in the group don’t understand.
“See, they don’t know,” Skeen tells him. “They got no idea.”
Then he tests Cassian with the second tattoo. And since Cassian recognizes them both, he asks, “So, where were you?”
“Sipo. Youth center,” Cassian replies back.
When Skeen says he never heard of it, he remarks, “They built a lot of cages, huh.”