Immutable means "can not change". When an object is referred to as Immutable it means the internals cannot be changed i.e. it's member variables. This is a design choice, where you might wish objects to be passed around as method arguments but you do not want the original object's internal values to change. Remember that in C# when you pass an object of type Reference Type into a method as an argument, you are actually passing it by reference, meaning the method can change the original objects values. To protect objects from this make them immutable. How: Objects are made immutable by simply making their internal variable readonly.
Layman explanations of Software coding and configuration techniques. With example code where possible.