Its basically saying that any class that is instantiated has an implicit constructor:
public class B {
//constructor
public B() {
//implicity constructor
}
}
public class A {
//constructor
public A() {
Bb = new B(); //calls the constructor inside B during setup even if the constructor method does not exist within B an implicit constructor is made
}
}
The default constructor is the no-argument constructor automatically
generated unless you define another constructor. It initialises any
uninitialised fields to their default values. link
When B is instantiated from A this constructor is called during its creation basically. For more specific details you should really ask in a different exchange than stack-overflow maybe try the programmers section.