To be in sync comes from synchronized. "We synchronized our watches so that we would both be at the trains station at exactly 10:00." = We made them the exact same time. To be in sync means to be going in the same direction or going in harmony with another thing, especially when it comes to being modern or up-to-date. He is in sync with the rest of the group. = He has the same opinions or works well with the rest of the group. My best friend and I are always in sync. Whenever she wants to stay home and chill, I do too. And when it's time to go out to the clubs for some fun, we are both in the mood for it. Her ideas of parenting are out of sync with modern ways of raising children. = She raises the children like they did in the past, not in the way they do today. Parents are often out of sync with their teenagers. = They don't understand their teens or their world. They don't know how to relate to their teens. Her hairstyle is out of sync with the modern styles.