You've left some portion of the VLOOKUP formula out of your post, but
essentially the formula says, "in a particular sheet, look in column A
for a particular value and when you find the first exact match, return
the value in column L from that row".

FALSE in that context is an parameter of the lookup that returns only
an exact match.

Your post doesn't mention what value is returned for that VLOOKUP, but
if it is #REF it can mean that the tab from which the lookup is pulling
the data has been deleted.

Dave O