This feature is useful when restricting the form's access to certain email addresses.
When this option is enabled, visitors must either authorize using their Google account or enter an email to which a verification code will be sent. Only verified emails matching the allowed keywords can view and fill out the form. You can add multiple keywords to the allowed list.
For example, if you want to restrict access to the form for your company's employees and you know each of your employees has an email with the domain @acme.com, then you can add @acme.com to the allowed keyword.
How to enable this option
Open the form and click on the Settings tab.
Click on Access control in the left sidebar.
Select Only the people with emails containing certain keywords can submit the form.
Type the keyword in the Enter keyword field. Click Add allowed keyword to add another.
Click Save changes.
Publish the form so the new setting takes effect.
To explore the other access control options, see Managing access to a form.