The idea
Two hundred people at a wedding
Stand on a balcony and look down at the crowd. You cannot read anybody's name badge, but you can see the clusters: the bride's colleagues here, the groom's cousins there, and two or three people who keep moving between them.
The clusters are communities. The people crossing between them are the bridges you measured last chapter. The repeated little shapes, one person holding court over four listeners, are motifs.
This chapter names those three things and the tools that find them.