Loads of Royals are said to be in line to the throne, the line is like a queue to rule the country. Where you are in the queue depends on who your parents were.
Prince William is younger than many people behind him in the line for the throne.
He is ahead of them because his dad, Prince Charles, was first in the queue.
If someone in the queue has a baby their child is not sent to the back of the line. Instead they slot in just behind their dad or older brothers.
These are called the rules of succession.