You are able to set if the admin of the site needs to approve a member when they sign up by selecting this option in the Levels section.
You can click the Edit option for the Level and then select the Require Admin Approval After Registration option of your choice.
One option is for free levels and the other is for paid levels.
Once this has been set, the admin must approve any users registering for that Membership Level before that user can access any content.
This is the process that happens if a membership level is set to require Admin Approval.
Similar to requiring Admin approval, you can also select Email Confirmation if you choose to.
This will require the member confirm the registration through an email that is sent to them before they can login.
You can approve the member within the Members tab.
Here is a screenshot with numbered steps that should help: