What is meant by "churn"?
"Churn" in the most common usage is the rate that existing members of a group leave the group (for example, customers of a company stop being customers- if a company has 15% annual churn, 15% of its customers leave every year).

In this context, then, it would seem that the author is saying that you have to understand the rate at which records may be deleted or otherwise removed from tables in order to understand how to best plan the maintenance of the system.