What does it mean if the circumference of my baby's head is above a certain percentile?

See below. As long as your baby's head stays at that percentage there is not problem. Some children have large heads, and some small. If the head circumference keep increasing then there may be a problem. The pediatrician is well aware of this and will keep an eye on it for you.
Depends. It depends on which percentile and what the other ultrasound findings are. If the brain anatomy looks normal otherwise, probably nothing to be concerned about. Realize that ultrasound isn't exact either.