Underwriting is the guarantee provided by the bank to the issuer (or seller) that the deal (be it an issue of securities or a Syndicated loan) will find buyers/lenders at the agreed upon price. By underwriting an issue, the bank assumes a Risk of a failure of the transaction.

(See Chapter 26 Value and corporate finance of the Vernimmen)

To know more about it, look at what we have already written on this subject