[Javascript] code deciphering help

[David Levy]

Hi, I was wondering if anyone can make sense of this little snippet of code:

y=(nav)?e.pageY:event.y+document.body.scrollTop;
skn.top=y+yyy;

my experience with javascript is very limited.  Here's what I do know.  nav is a boolean variable, I believe skn is an object, and I understand that there is some sort of assignment going on.  What primarily confuses me is the question mark(?) and the colon(:).  The code comes from a script written by Mike McGrath.  Thank you to anyone willing to take a look.

David Levy
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