When a customer registers, the New Registered User email contains a validation code and the customer is taken to the Email validation page to enter it. The code is a 6-digit number, valid for 5 minutes. If it has expired or the customer never received it, the page's resend link generates a new code and sends the New Email Verification Code Sent email (We sent a new validation code to your email and it's only valid for 5 minutes). Once the code is accepted the account is marked validated and the customer continues to the page they came from.

If you do not want this step, set Disable Email Validation Security to Yes on the Business: Email tab of Client Config; the registration email then simply links to the site instead of showing a code.