When you're in arrears, you owe someone money, and you've missed a payment deadline. If you don't make your car payment for a few months, you're in arrears.
In arrears is a legal term for being behind on a debt — either you haven't paid it back when you promised to, or you've missed a monthly payment or two. A landlord can legally evict a tenant who's in arrears with their rent payment, and a bank can charge fees and hurt your credit if your loan payment is in arrears. In arrears comes from the Middle English arrear, "behind or overdue," from a Latin root.