'Cash purchases' means any payment 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.
Examples of the types of supplier from whom you might make Cash purchases could include new suppliers that you have not yet established a track record with, and other suppliers that you only use from time to time.
Cash purchases only include items that you buy for resale. 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.