An ACH return is a credit or a debit entry typically initiated by a Receiving Depository Financial Institution (RDFI). The RDFI will submit the return to the Originating Depository Financial Institution (ODFI) for a variety of reasons (which can be seen here).
If your bank receives a return, you will be able to see these highlighted within Modern Treasury. These returns will often include a reason for why they happened, which you can view under the returns section.
Understanding the reason why a payment failed can allow you to work with your counterparty to resolve the underlying issue and reinitiate a successful payment.