Answer
Petty cash is a small amount of cash kept on hand to pay minor expenses (postage, refreshments, small stationery). The petty cash book records these small payments, usually with analysis columns by expense type, and receipts of cash to top up the float. It's a book of prime entry (and often a ledger account for petty cash).
Real-world example Small office expenses like stamps and taxi fares are paid from petty cash and recorded in the petty cash book.