As mentioned in the Customers Tab Overview, the Customers > Pending Approval Tab is only visible when the "New customer accounts require approval?" setting is checked in the Account > Store > Frontend Tab.
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This page will allow you to approve or unapprove pending customers.
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Clicking the thumbs up icon will approve the pending customer, moving the customer over to the Active Customers Tab.
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Clicking the thumbs down icon will unapprove the pending customer, moving the customer over to the Unapproved Customers Tab.
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You can also click the Red circle with an X icon to delete the customer account.
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Similiar to the Active Customer Tab, clicking the edit button to the right of the customer's name you can view your their account information, orders history, store credit, physical address information, and survey answers.

For an overview of the Customers Tab Click Here.