what does it mean Define the Normal Flow in clean code book?

I am reading clean code which is written by Robert C. Martin in the 7 section about error handling There is a piece of code that I could not understand it.

try {
 MealExpenses expenses = expenseReportDAO.getMeals(employee.getID());
 m_total += expenses.getTotal();
} catch(MealExpensesNotFound e) {
 m_total += getMealPerDiem();
}

what does it mean? and it is written that we need to refactor it via special case pattern like the following:

public class PerDiemMealExpenses implements MealExpenses {
 public int getTotal() {
 // return the per diem default
 }
}

Can you translate it in a simple way for me? thank you in advance.