This article contains plot details from the latest book.
A null is a type of elvin invention that blocks signals. Tinker made one for Sophie in Flashback that looks like her Ruewen Crest. When Dex asked how Sandor would be able to track her with the null, Tinker replied that the null was capable of blocking certain signals and allowing other ones. In Flashback, the null blocks the signal from Alvar's tracker, preventing them from knocking Alvar out.