If the eldest daughter had female children then her children would control the throne.
Elizabeth has Anne (and no Charles, Andrew or Edward or their heirs) and or a Alexandra (or more daughters). Then Anne becomes Queen. Anne has two children (Peter and Zara). Peter becomes King, his heirs, then Zara and her heirs.
If Anne had two daughters (say a Zara and a Phillipa), then eldest daughter Zara only had daughters but they are still ahead of the younger daughter Phillipa and her successors. So if Anne's younger daughter Phillipa had a son, he would be behind his mother the Queen, Aunt Zara, her daughters (and their kids) but he would jump ahead of any of his sisters.
Think of Queen Mary II and Anne. They both died without surviving heirs. The throne went to the heirs of the Electress Sophia, who was the eldest daughter of the eldest daughter of James I of England. So the House of Stuart:
James I > Charles I > Charles II > James II > Mary II > Anne II end of the House of Stuart because Anne has no living heirs
James I was the father of Charles I and Elizabeth who was the mother of Sophia, whose son became George I and so started the House of Hanover
I hope I got the names and placement right but that is the general idea. Its important to note that just because you have sons (you don't jump head of people). Thus the current Edward, Duke of Kent or the late Prince William of Gloucester did not jump ahead of Queen Elizabeth II and Princess Margaret when they were born.