Payment terms are the line on the invoice that says when the money is due. On receipt means pay as soon as it arrives; 14 days (or net 14) means fourteen days from the invoice date; net 30 is thirty days, the most common terms between businesses. Whatever you choose, print the terms and the actual due date next to them, so nobody has to count.
If you agreed nothing, UK law fills the gap: between businesses an invoice is treated as due 30 days after the customer receives it, or the goods or service if that is later, and the Late Payment of Commercial Debts legislation lets you charge statutory interest at 8 percent above the Bank of England base rate, plus a fixed recovery fee, once it is overdue. That is a description of the rules rather than legal advice; most small businesses never charge the interest, but knowing it exists changes the conversation.
A sentence that works on most invoices: Payment due within 14 days of the invoice date by bank transfer to the account below. Please quote the invoice number.
In FullDiary, the Due box on the invoice form offers On receipt, 7, 14, 21 or 30 days, a custom date, or no due date at all. An invoice due on the day it is issued prints "On receipt" rather than a date.
Set your normal terms once in Settings, under Business details, and every new invoice starts with them - if all your work is payable on receipt, you never touch the box again. You can still change it on any single invoice.
The due date is what the reminders run off: on the Pro plan, FullDiary chases an unpaid invoice the day after it falls due and again a week later.