A CUSIP number is a security's unique identifying number, not unlike a product's catalog number.
CUSIP numbers are like bar codes. Just as it's easier to swipe a bar code at the hardware store checkout than to search for the price of a 40-count box of Wazzuzle brand widgets, it's easier to handle market deals using CUSIP numbers. They provide uniformity and efficiency in a fast-paced, jam-packed, computerized world.
The nine digits that make up the CUSIP number are assigned by the Committee on Uniform Security Identification Procedures (CUSIP). The first six digits identify the issuer, such as a company. The next two digits identify exactly what kind of security it is, such as a particular kind of stock or bond. The final digit acts as a "check digit," utilizing a mathematical formula to verify the accuracy of the other eight characters.
It's good to have CUSIP numbers handy when you're transferring shares of stock or if you've lost your stock certificate(s).
CUSIP membership gives access to all numbers. Fidelity Investments provides a search engine that lists CUSIP numbers.