First of all, banks would only lend against collateral (secured lending) rather than lending directly to another bank  3  6  Debelle G (2010), “On Risk and Uncertainty”, Address to Risk Australia Conference, Sydney, 31 August. BIS central bankers’ speeches  without any security (unsecured lending).
In plain English, this means that banks would only lend to another bank if the borrowing bank would provide something like a government security or a portfolio of mortgages as collateral to the loan.