Let's look at line 9 of your code:
Dog d = new Dog();
You are declaring a
variable with the name 'd'.
Variables have a
type. The type of the variable 'd' is 'Dog'.
Dog is a
class. A class is a 'template' from which you create
objects. On the right side of line 9 (after the =), you create a new Dog object. Then you assign that to the variable 'd', so 'd' now refers to the new Dog object.
After that, you use the variable 'd' to do things with the new Dog object that you created. For example, in line 10, you set the value of one of the properties of the Dog object.