ssrs 2008 r2 iif with a lookup

In a ssrs 2008 r2 report, I have found the following expression and I would like you to explain the following code to me:

=IIF(Lookup(Fields!SK.Value, Fields!SK.Value, Fields!StCount.Value, "DS_StCount") = Nothing, 0, Lookup(Fields!SK.Value, Fields!SK.Value, Fields!StCount.Value, "DS_StCount"))

I know that a lookup function compares the first values to see if they are equal, and if so they value in the third column is returned. However I do not know what this entire

iif statement is doing. Can you explain it to me?