The first place to start with a question like this is to think of the events that occur in these Chapters and to consider how they relate to the overall theme of the novel and the plot. Chapter 18 and 19 therefore relate how Kit was arrested for being a witch and helping Hannah Tupper to escape. It narrates how she was placed in prison and also her trial and how it is proved that she is not a witch. For me, one of the most important passages is given in Chapter 19, just after Prudence Cruff reads from the Bible to show and prove what she had been doing at Hannah's house:
In the warm rush of pride that welled up in her, Kit forgot her fear. For the first time she dared to look back at Nat Eaton where he stood near the door. Across the room their eyes met, and suddenly it was as though he had thrown a line straight into her reaching hands. She could feel the pull of it, and over its taut span strength flowed into her, warm and sustaining.
This passage is important for a number of reasons, but chiefly because it comments on the relationship between Nat and Kit. Nat has risked a flogging to come and help save Kit, and as Kit looks at Nat, the connection that is described foreshadows the happy ending of their tale and settles once and for all the question of who Nat loves and who Kit will marry.