The narrator explains that the first week of August is the highest point of the year. The months before are just spring, while the months after represent a decline towards fall. That week in August, however, is hot, silent, and still. The lightning at night is just lightning, with no rain or thunder. During this time, people do things they're guaranteed to regret later.
By beginning with an explanation of the time of year and where August fits into the cycle of the year, the narrator situates nature and these natural cycles as a unifying motif of the novel.
The narrator says that one day during this time of year, three things happened that seem disconnected but actually are very connected. First, Mae Tuck left for Treegap to meet her sons, Miles and Jesse. Winnie Foster, whose parents own Treegap wood, decided to think about running away, and a stranger (the man in the yellow suit) showed up at the Fosters' gate. The narrator says that the wood is the hub of a "wheel" that connects these people. It's important to leave the hub of wheels alone, as they hold things together.
Here, by connecting three seemingly unconnected people to each other by way of the wood, the narrator again situates nature as a central, unifying force for all life. By saying that it's best to leave the hub alone, this moment also sets up the novel's central conflict as being a question of preserving the wood or altering it, which the narrator suggests will bring only bad things.