Strong ties are firsthand relationships, so friends, coworkers, and people you have direct contact with on a day to day basis. These are people you know, not people you know of.
“In general, […] most of people have friends who are like themselves. People might have some acquaintances who are richer, a few who are poorer, and a few of different races — but, on the whole, our deepest relationships tend to be with people whoLook like us, earn about the same amount of money, and come from similar backgrounds.”
Strong ties create high clustering in social networks—your friends tend to know each other, forming tight-knit groups. While this clustering might seem limiting, it’s actually essential for cooperation to emerge. In highly clustered networks, people interact repeatedly with the same small group, which fosters trust, accountability, and cooperative behavior. The paradox of social networks is that you need both: strong ties to create the clusters where cooperation thrives, and weak ties to create the shortcuts that make the world small.