Magento 2 gives us the ability to take more control of the customer account creation process. There is a special configuration setting located in admin under Stores → Configuration → Customers → Customer Configuration → Create New Account Options named Require Emails Confirmation (Yes/No)
If you set this setting to Yes, a customer will not be able to log in without account confirmation. He needs to click the activation link in account confirmation email.
By the way, this email template is defined in vendor/magento/module-customer/etc/email_templates.xml file:
<template id="customer_create_account_email_confirmation_template" label="New Account Confirmation Key"file="account_new_confirmation.html" type="html" module="Magento_Customer" area="frontend"/>
Unfortunately, there is no way to confirm user account from Magento 2 admin area.
If you are a developer and you are doing some local testing (assuming you don't receive emails from your local installation) you can confirm a customer account via the database.
There is a field `customer_entity`.`confirmation` which contains activation hash code.
All you need to do - is simply remove that hash value.
Here is SQL query that will help you to confirm customer by email:
UPDATE customer_entity SET confirmation = NULL WHERE firstname.lastname@example.org';