Generation is one of those words that have a lot of different meanings. The two most common involve groups of people and some things they have in common.
One type of generation involves members of your own family. You and any of your brothers and sisters, your first cousins (your aunts’ and uncles’ children,) second cousins your great-aunts’ and great-uncles’ grandchildren,) and anyone else on your level of the family tree is part of the same generation. Your parents and their first, second, and third (and so on) cousins are part of a generation too. If someday you have your own children, your children and your cousins’ children will be part of the same generation. Generations might be able to be traced back hundreds of years, depending on if good records were kept of family histories.
Another type of generation goes beyond your family. It involves anyone who is about your same age. This type of generation does not have defined rules, but it involves people who are alive at the same time and who experience the same events. People who were kids when the Harry Potter movies first came out are part of the same generation. People who were old enough to join the army during the Iraq war are part of the same generation. The shared experiences can be anything. Every one who is a kid right now is part of your generation. Think about the experiences you share that today’s grown-ups did not experience when they were kids.