Shopify does not offer an email verification setting for customer emails, as is customary in most other places on the web. By default, when a customer registers for an account on the storefront, they immediately gain access to their account. This presents a problem for certain members-only stores or wholesale businesses.
However, if someone tries to create an account using an email address that is already liked to a customer account (whether through email subscription, previous purchase, or other means) that customer is required to verify their email before creating an account in order to avoid a malicious party gaining access to someone else's order history (or other information) without actually having access to that email address.
I assume that has something to do with the "verified_email" field.