Simply put, in the book catalog! $A$2:$B$9 looks up D4 in this region and returns the value of D4 in the second column of the row where this region is located.

For example: A3 = D4 in this area, then B3 is returned (A (column A is the first column, column B is the second column, and the second column is returned).

FALSE means accurate search, which can be omitted, because the default is accurate, so it can be changed to =VLOOKUP(D4, book catalog table! $ A $ 2:$ B $ 9.2，)