What does Petty cash book mean? why ?

The petty cash book is a formal summarization of petty cash expenditures, sorted by date. In most cases, the petty cash book is an actual ledger book, rather than a computer record. Thus, the book is part of a manual record-keeping system.

And there are two primary types of entries in the petty cash book, which are a debit to record cash received by the petty cash clerk (usually in a single block of cash at infrequent intervals), and a large number of credits to reflect cash withdrawals from the petty cash fund. These credits can be for such transactions as payments for meals, flowers, office supplies, stamps, and so forth.