Ccritical infrastructure (CI)

What does critical infrastructure (CI) mean? What are soft and hard targets? What does the term cascading effects mean? What is the most important CI where you live or work? Why do you say this?

TEXTBOOK: Bennett, B. T. (2018). Understanding, assessing, and responding to terrorism: Protecting critical infrastructure and personnel (2nd ed.). Hoboken, NJ: John Wiley & Sons, Inc. ISBN: 9781119237785.