OKAY, So when Kate Middleton was pregnant, there was a law passed that meant if the child was a girl, it would be able to be queen.
But England has had quite a lot of queens. What exactly did that law mean and how did past queens become queens if it wasn't possible for females to inherent the title until recently?
Women weren't barred from becoming queens, they were just passed over if there were other male heirs. For example Queen Mary I and Queen Elizabeth I have a younger brother named Edward. Edward became king first even though he is younger than both Mary and Elizabeth. Edward died young and there weren't other heirs except Mary and Elizabeth so each of them became queens.
The Succession to the Crown Act removed the "Male-preference primogeniture" rule. In other words, before that Act passed, all male children of the heir to the throne would be in line of succession before the female children. This is why the Princess Royal (Princess Anne) is behind her younger brothers, the Dukes of York (Prince Andrew) and Wessex (Prince Edward), in line of succession. Had Prince Charles died before having children, then Andrew would be heir to the throne, not Anne. The act is not retroactive, so Anne remains behind her brothers (currently 12th in line).
Under the new Act, Princess Charlotte will retain her position as (currently) 4th in line, even if Prince William and Kate have another son. She's still behind her brother, George, however.
The Telegraph has a good article on it, with infographic.