Explain the role of the Bank for International Settlements as the central banks' bank?

What will be an ideal response?

One of the BIS's main functions is to serve as a bank for the world's central banks. It does so by helping some 140 central banks manage and invest their monetary reserves. Most of these funds are placed in the world's money market in the form of commercial bank deposits and short-term negotiable instruments (such as certificates of deposit). Beyond placing surplus funds in the international marketplace, the BIS occasionally make liquid resources available to central banks. Such transactions include swaps of currency for gold, credits advanced against a pledge of gold or marketable short-term securities, and, less frequently, unsecured credits and standby credits. The bank also carries on exchange transactions in foreign currency and gold both with the central banks and with the markets.