It can be understood as a variety of forms of things.
The manifestation of polymorphism:
A reference to a parent class points to its own child class object.
A reference to a parent class can also receive its own child class object.
The premise of polymorphism:
It must be a relationship between classes, either inheritance or implementation.
There is also a premise: there is coverage
Benefits of polymorphism:
Greatly improved the scalability of the program
Disadvantages of polymorphism:
Improved extensibility, but only the reference of the parent class can be used to access members in the parent class.