What does it mean to “build white space” in your calendar?

In order to minimize the number of interruptions that may interfere with staying on schedule, calendar space should be left deliberately unfilled to accommodate unforeseen delays and emergencies.

An example of “white space”  is when a calendar schedule is about 80 percent capacity for in-office and about 50 percent capacity when returning from a trip, or about 40 hours of work for a 50-hour work week, knowing that another 10 hours of “stuff” will appear.