Investing in margin is borrowing cash to open a new position. If the aggregate cash balance in a given account is a debit (negative balance), then funds are being borrowed and the loan is subject to interest fees. Note that even if the aggregate cash balance is a credit, a loan may still remain resulting in balance netting or timing differences.

For additional information, please see How to determine if you are borrowing funds from IBKR in the IBKR Knowledge Base.