$iif() is just like IF, but on one line.
$iif(something compared_to something,true,false)
So, //echo -a $iif(1 > 2,It's greater,It's not greater)
That would echo the "false" item, "It's not greater", because the comparison is false.
So he's just checking if *_* iswm the nick and if it is, do something, if not do something different. If you want multiple commands, don't use $iif... just use a normal IF.
Again, you can see 2 examples in your other thread.