Manual-pay is the opposite to Auto-pay. Payment will only ever be made once you have clicked the 'pay' button. Either Manual or Auto pay can be set for each supplier, it's up to you.
For example, you might decide that some suppliers should be set to Auto-pay, such as TV License, and others set to Manual-pay, such as energy. This way you only have to remember to pay the ones you want to keep an eye on.
You can set all payments to Manual-pay if you want, you will still get notifications and reminders the help you stay on top of your bills, however, remember to click the pay button otherwise the bill will never be paid.