A phonecall to their bookkeeper might help explain it. You sure want to make sure you have all the money they owe you.

Or

If you look and it says you owed them something it is a nice way of saying they messed up and you owe them more money it is pay you should have given them back when they didn’t take it out of your check. Backpay.

is a payment due for work done prior to the current pay period. Back pay may be the result of a missed payment or a payment that was intentionally not made, or back pay may be owed due to a backdated pay increase. Back pay applies to both hourly as well as salaried employees, and back pay may also be applied to over-due payments owed to freelance workers, part-time employees, as well as consultants. If a company is chronically late in the payment of wages and salaries and owes considerable back pay to its employees, that is often a sign that a company is in financial trouble. Back pay may also be referred to as back salary or back wages.

Depending on if it was a + or -, it could have been for many different reasons.