What does $ signify in internal code of inner class in java?

Question

I read that this code is internal code generated by compiler for a member inner class in java. What does this$0 signify in the given code?

import java.io.PrintStream;
class Outer$Inner  
{  
    final Outer this$0;  
    Outer$Inner()  
    {   super();  
        this$0 = Outer.this;  
    }  
    void msg()  
    {  
        System.out.println((new StringBuilder()).append("data is ")  
                .append(Outer.access$000(Outer.this)).toString());  
    }
}

I read this code on https://www.javatpoint.com/member-inner-class