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 the paid Production plan. When creating a new account, you need to request access to this plan by filling out an access request form. The Production plan allows verification of an unlimited number of client accounts. Plaid’s 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 its 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.