Buffer days are used to prevent a subscriber's first and second subscription order from falling too close together when you've configured your monthly subscriptions to bill on the same day every month.
These aren't always necessary, it depends on the subscription model, but for most store owners buffer days are important to prevent double billing. Typically we recommend 15 buffer days, but it can vary across different use cases.
If a customer's first order was on the 31st of the previous month, and the monthly date is the 5th of the month, you most likely would not want a subscriber to have a second order generate on the 5th.
Buffer days are the number of days leading up to your fixed monthly re-billing date, where if a customer's first order fell on one of those days, they would not have an order generate on the upcoming re-billing date. They would not have another subscription order generate until the next month.
If you configure your fixed monthly re-billing date to be the 1st of every month and you set your buffer days to 15, this means any first subscription order that is placed in the 15 days leading up to the 1st of every month will not have an order generate until the following month.
First Order: October 25th
Second Order: December 1st
The October 25th Shopify order would be designated as a November 1st subscription order so that the October 25th order would count towards November 1st. The second order would be automatically scheduled on the next cycle date of December 1st.