What is the dependency inversion principle and why is it important?

Question

What is the dependency inversion principle and why is it important?

Answer 1

If we can take it as a given that a "high level" employee at a corporation is paid for the execution of their plans, and that these plans are delivered by the aggregate execution of many "low level" employee's plans, then we could say it is generally a terrible plan if the high level employee's plan description in any way is coupled to the specific plan of any lower level employee.

If a high level executive has a plan to "improve delivery time", and indicates that an employee in the shipping line must have coffee and do stretches each morning, then that plan is highly coupled and has low cohesion. But if the plan makes no mention of any specific employee, and in fact simply requires "an entity that can perform work is prepared to work", then the plan is loosely coupled and more cohesive: the plans do not overlap and can easily be substituted. Contractors, or robots, can easily replace the employees and the high level's plan remains unchanged.

"High level" in the dependency inversion principle means "more important".