The Renewal Period or Renewal Frequency is how frequently the vendor/service charges you. It varies between monthly, quarterly, or yearly based on your subscription to the vendor/service.
If you pay the vendor/service once a month, then the Renewal Frequency is set to 1 month, for a yearly subscription, the Renewal frequency is 12 months.
Say, you pay Slack $100 every month, you can create a Recurring Virtual Card and set the Renewal Frequency as 1 month and Spend Limit as $100. If the current month is October, Slack can charge you $100 in October and charge again in November.
Airbase will send reminder emails if the Recurring Period is set over three months.
However, Spend limit resets differently from the renewal period.