Long-term upside is another way of talking about a prospect's ceiling.
A ceiling is the best possible player a prospect can become. But it usually takes a while to get there, which is why we call it long-term upside.
Though these players have the highest ceilings and upsides, it doesn't mean they are the six best prospects. Victor Oladipo is likely to get selected before Alex Len, though if they both reach their ceilings, Len will end up the better player.
These are the guys who could turn out to be the top players from this draft if they all reach their NBA ceilings.