Mortgage payments are paid in arrears. That is, the interest due for a period is paid at the end of the period, as opposed to the beginning of the period.
For example, if you have a mortgage payment due on June 1, that payment is for the interest accrued during the month of May, along with the contribution towards loan principal and any escrow items due.
In contrast, rent payments are made in advance. If you have a rent payment due on June 1, that rent payment is for the month of June.
Arrears also refers to debt or loan obligations that are overdue. For example, if you are two months late on your mortgage payment, your loan would be “two months in arrears.”