The Fee Rate represents the indicative rate (generally expressed as an interest rate) that the lender assesses to the borrower of the shares. This is the indicative borrow fee rate that clients will pay IBKR to short a stock. It is a debit from the account.
The lender will then reinvest the cash collateral posted by the borrower to secure return of the shares and may offer a partial rebate of those earnings back to the borrower's broker (generally equal to the Fed Funds Rate in the case of USD stock loans). Many brokers do not pass any part of this rebate to retail accounts, although IBKR does.
More information on securities financing interest rates can be found on our website.