In Java, a constructor is a special method used to initialize objects of a class. If a class doesn't have any constructor defined explicitly, the Java compiler automatically creates a default constructor for that class.

The default constructor in Java is a no-argument constructor that initializes the object's fields with their default values. The default constructor has the same name as the class and is called when an object is created using the new keyword without any arguments. For example, if we have a class called Person, the default constructor would look like this: 

public class Person {
   public Person() {
      // Default constructor code goes here
   }
}

Here, the default constructor initializes the object's fields with their default values, such as null for object references, 0 for numeric types, and false for boolean types.

It's important to note that if a class has at least one constructor defined explicitly, the Java compiler doesn't create a default constructor. In this case, if we try to create an object using the new keyword without any arguments, a compilation error will occur.

In summary, the default constructor is a no-argument constructor that is automatically created by the Java compiler if no constructor is defined explicitly.