When you say that someone is ‘grounded’ what you mean is that he is very sensible. Though the individual has everything, and has achieved many things, he behaves in a sensible fashion. In other words, the individual has his feet on the ground.
This is one of the meanings of the word.
Examples: ”The kid’s father is a business tycoon and his mother is a famous actress. You certainly can’t expect him to be grounded.” * “Suresh has everything that one can ask for, but he seems fairly grounded.” * “The billionaire’s wife makes sure that her children stay grounded.