'Cash from debtors' means any payments you receive from customers for some time after your performances - for example in the following month. While you are waiting for payment from them, they are your debtors. When they pay you, the payment is referred to as Cash from debtors.
You may not have a significant number of debtors if any, but you might do some work - such as playing in a pub every Friday evening - that you get paid for each month. If you think you are going to receive any Cash from debtors, enter the amounts in your cash flow in the months in which you will receive them.
Your business stationery
Have some invoices printed for your customers. A good printer will be able to advise you on layout, style and even content. If you are registered for VAT you must include your VAT registration number on your invoices.
A business customer such as a pub may well ask you for an invoice to prove that the expense is a legitimate one and not cash going straight into the business owner's pocket!