In a nutshell it means.. You are trying to access an object without instantiating it.. You might need to use the "new" keyword to instantiate it first i.e create an instance of it.
For eg:
public class MyClass
{
public int Id {get; set;}
}
MyClass myClass;
myClass.Id = 0; <----------- An error will be thrown here.. because myClass is null here...
You will have to use:
myClass = new MyClass();
myClass.Id = 0;
Hope I made it clear..