What does low-key mean?

Inside the Rolls-Royce Phantom, Su Chengru's face was filled with excitement. He touched things here and there, looking around. He had never ridden in such a luxury car before.
Until now, it feels like I am in a dream and I can't believe it.
"Mr. Chen, Uncle Su, we are about to arrive at the treasure appraisal meeting."
Wang He, who was sitting in the front passenger seat, said respectfully, "My father has already made arrangements for today. After you two enter,...