Link churn which is mentioned in this patent application is "computed as a function of an extent to which one or more links provided by the document changes over time." Basically that means the rate a link and/or its anchor text change from an origin's page or document. The patent application discusses that Google may apply a penalty of your "link churn" is above the threshold, i.e. "63. The method of claim 62, wherein adjusting the ranking includes penalizing the ranking if the link churn is above a threshold."