Not able to understand code. Please guide me

public  abstract class Class1
  {
      protected double a;

      public virtual double Getval()
      {
          return this.a;
      }
  }

  public class Class1A : Class1
  {
      public Class1A()
      {
          this.a = 6.99;
      }
  }

  public abstract class Class1B : Class1
  {
      protected Class1 Obj;
      public Class1B(Class1 instance)
      {
          this.Obj = instance;
      }

      public override double Getval()
      {
          return (this.Obj.Getval() + this.a);
      }
  }

  public class Class1B1 : Class1B
  {
      public Class1B1(Class1 tbd)  : base(tbd) // What is this? please explain
      {
          this.a = 0.99;
      }
  }

public class Class1B1 : Class1B
    {
        public Class1B1(Class1 tbd)  : base(tbd) // What is this? please explain 
        {
            this.a = 0.99;
        }
    }