They help people know when to shake hands, where to sit, how to converse, and even when to laugh. We relate to others through a shared set of cultural norms, and ordinarily, we take them for granted.
This part of the textbook helped me remember one of the norms that are in my own life. Whenever we arrive at an event, for example, like a birthday party or just a family gathering, you always have to say hi to everyone, whether that is a handshake or sometimes even a big hug(usually a hug). Even the guests that you don't know, if you don't, it can be seen as rude or impolite. If you just arrived, they are expecting you to come up to them and greet them, since they're already at the party; they just wait for you to approach them first. This is just a cultural norm for us. My parents were the ones who told me this is how it is practiced, so they'd always say, "Go say hi to your aunt," and if I didn't, my aunt would come up to me and say, "Where's my greeting??"