A Recurring Invoice is an invoice that is sent to your customer in regular intervals of time.
To create a recurring Invoice, select Yes when it is asked- "Do you want this invoice to repeat in future?" before saving the invoice. After selecting yes, you need to fill three fields:
1) Schedule on - Schedule the date of invoice.
2) How frequently - Frequency of invoice: daily, weekly, monthly, quarterly, half yearly or yearly.
3) Repetitions - Number of times they want to repeat an Invoice, by default it is set to Infinite.