Plaid.com is a third-party service that partners with Stripe to allow you to verify your clients' bank accounts for ACH transfers. Your business can sign up for Plaid’s verification services through multiple plans. The Development Plan is a free plan that allows verification of up to 100 client accounts. When creating a new account, you will need to request access to this plan by filling out a brief request form. There is also the Production plan, which is a paid plan that allows verification of an unlimited number of client accounts. This plan also requires you to request access. Plaid’s response to the Development plan is ideally instant while its response to the Production plan might take up to a few days. ACH transfer payments cannot be initiated until Plaid accepts your request and activates the necessary keys for use.
Your clients do not need a Plaid account, but they use their interface to log in to their bank accounts and allow Stripe to set up the ACH transfer payment for the selected account. Check here for details.