What does webdriver driver means?

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Java

OOP

selenium

I understand that webdriver is an interface of which driver is a reference variable. But in Java the reference variable is not properly defined. It holds a reference the object. It does not hold the memory address of the object but a reference. Now webdriver is an interface and it's refernce variable is driver which will refer the object of Chrome driver. So if it is only going to point to the chromedriver, then a chromedriver object refernce can do that. It's not like it will hold the properties of webdriver interface, chrome driver class is already doing that.

What I have tried:

Can someone explain this concept to me and please don't post interface or top casting links? I am aware that this is top casting i am just trying to understand the purpose of this line here.