Lookup Visible values on another sheet

[jomili]

I found a formula online that does what I need (pulls in the visible values from another sheet) but there was no explanation of how the formula worked. I was able to modify it to work for what I needed, but I'd like to understand it so I can use it in other applications, so I'm hoping someone will have pity and explain the formula to me. "KeyValues" is a named range, corresponding to column F on the "Enter FTEs" tab. See the attachment to see the formula in operation. The formula in question is reprinted below.

=IFERROR(LOOKUP(2,1/((COUNTIF(A$3:A3,KeyValues)=0)*(KeyValues<>"")),KeyValues),"")

Thanks,
John