You have the option to specify whether a particular user requires approval before they can send a document, decide who their manager is and allow that manager to approve or deny any approval request.

The first step is to set this up under User Permissions which you can access by going to Settings > Users and Teams > Permissions. For any user who requires approval, select 'Sending Approval'. This will then show you a drop down which allows you to select which other user should review their proposals.

Any user who requires approvals or is set as an "approver" will see a new tab on the proposals list for "Pending" documents. This will show any documents that are under review, have been approved for sending or denied with feedback.

Read more about approving or denying a document, or find out what happens when you request approval of a document.