Q:

What does it mean when a C# function parameter contains `= null`?

I'm going through an MVC tutorial and see this line of code at the beginning of a function: 
private void PopulateDepartmentsDropDownList(object selectedDepartment = null)

After testing it out, I can see that the function works, but I do not understand how the function parameter works.  What does object selectedDepartment = null do?
I have done a general internet search and have not yet been able to locate an  answer.
I guess my question really has two facets: 

What does the = null portion of the parameter do?
Is it something that can be done but not necessarily should be done?

A:

It means that that parameter will be null, unless you decide to pass something. So in other words, its optional.
It can be done, and there is nothing wrong with it. Its pretty common practice.

