'Cash purchases' means any payments you make to suppliers with whom you do not have an account. As you don't have an account, you will have to pay for the goods at the time when you buy them.
You might make Cash purchases from suppliers from whom you make one-off or infrequent purchases. For example, a replacement part for an unusual tool.
Cash purchases only include items that you buy for resale/hire. They don't include things like tea and coffee bought for use at work, or items such as business stationery. Don't include things like this here. They're covered elsewhere in the cash flow.
Remember to keep any invoices and receipts for Cash purchases safely, even if the amounts are small.