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 or services at the time when you buy them.
For example, you might only use a specialist disposal site for certain types of hazardous waste infrequently - it might not be worthwhile or even possible to set up an account. Note that some landfill sites operate on a cash payment only basis and don't offer account facilities even to regular users.
Cash purchases only include items that you buy to use for waste collection and disposal purposes. 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 all the invoices and receipts for cash purchases, even if the amounts are small.