A shake-up is a major change or upheaval. If there's a shake-up on your soccer team, you might suddenly find yourself playing midfield instead of your usual position defending the goal.
Whenever there's reorganization at a company, especially if lots of people lose their jobs, it's described as a shake-up. You can use this word for any similarly drastic change, like a shake-up in your morning routine after your family gets a new puppy that needs to be walked for an hour, or a shake-up in government after a major election.