Although the central banker didn't say so, this may ultimately compel him to resort increasingly to managing

Question:

Although the central banker didn't say so, this may ultimately compel him to resort increasingly to managing the money supply by managing banks' excess cash reserves-the stuff from which banks create loans.
a. What are excess cash reserves?
b. Why are they called "the stuff from which banks create loans"?
c. How would the central bank manage these excess reserves?