'Cash purchases' means any payments you make to suppliers with whom you don't 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 local car accessory shops, or online suppliers like professional eBay traders who don't offer account facilities.
Cash purchases only include items that you buy for use when valeting or working on a vehicle. 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 safely any invoices and receipts for Cash purchases, even if the amounts are small.