Action<T> or Action<in T>?

I was reading about Action Delegate on MSDN and so this under syntax

 public delegate void Action<in T>(T obj);

Than I looked in c-sharpcorner.com and it used this syntax

public delegate void Action<T>(T obj);   

As you can see there is no in before T.
Which syntax is right and what does that in mean?
EDIT: The same syntax used for Predicate.

Thanks.