That's how many more delegates Senator Obama needs to win the nomination.
According to my math just after the Pennsylvania primary, Senator Obama can win less than half of the remaining pledged delegates and still pick up around 170 more delegates. In other words, he can basically lose the rest of the primaries and still pick up 170-180 delegates. That would mean he'd only need around 110 out of the remaining 250 (approximate) uncommitted superdelegates.