If a member owes money, this will show in red under their picture on their profile. When a member owes, this means that there will be some charges on their account which are overdue and have not yet been paid. These will show under the Charges Awaiting Payment section of the Accounts tab.
To log a payment against the member's outstanding charges, click the [Add Payment] button to pull these charges through to the Point of Sale and add a payment.
To clear the outstanding charge, that is to take no payment and mark the charge as not needing to be collected, see the article Credit Notes: How to clear a charge
From the POS, the outstanding charges will show in a list. Choose which charges the member wants to pay for by clicking the green Click to Add button. Once you're happy with charges in the sale the total will show on the right. Click the [Make Payment] button and choose the method this member used to pay for the sale. Click [Complete Sale] to finish and you'll be redirected back to the member's profile. Navigate back to the member's Accounts tab, and confirm that the charges are now marked as paid and they are no longer owing.