what is Co-Varient and Contravarient in C#

// Assignment compatibility.  

string str = "test"; 

// An object of a more derived type is assigned to an object of a less derived type.  

object obj = str;  

// Covariance.  

IEnumerable<string> strings = new List<string>(); 

// An object that is instantiated with a more derived type argument  

// is assigned to an object instantiated with a less derived type argument.  

// Assignment compatibility is preserved.  IEnumerable<object> objects = strings;  /

/ Contravariance.            

// Assume that the following method is in the class:  

// static void SetObject(object o) { }  

Action<object> actObject = SetObject; 

// An object that is instantiated with a less derived type argument  

// is assigned to an object instantiated with a more derived type argument.  

// Assignment compatibility is reversed.  

Action<string> actString = actObject;