If, for any reason, a customer has disputed a payment, the customer's bank will review the case and ultimately decide whether or not the customer's dispute claim is legitimate or not. You as the merchant will be asked to submit evidence to show that you've charged the customer correctly and that the customer did in fact order and receive the product or service they ordered.
If the bank decides in your favour, you have nothing to worry about. However, if the bank decides to side with their customer, they'll perform what's called a Chargeback. This means the bank will "charge back" the money to the customer from your account. This will in first line be handled by SimplyPayMe's acquiring partners, who will subtract the full amount from money you are scheduled to receive to your account for payments made in the past two days. If you are a business with an excessive amount of disputes and chargebacks, the acquiring partners may want to put you on a rolling reserve.