What does "not a payoff" mean?

It's irrelevant if the box says $0, but if there was an amount there, it would be the amount that you currently owe. Suppose that box said $1000. That doesn't mean that you could just send in $1000 and your loan would be completely paid off. Additional interest may have accrued from the time that the statement was generated, and there may be other miscellaneous fees associated with finally paying off the loan.