Emails can be sent to users to have them approve or reject directly without having to login to the UI to move a pipeline forward.
This page outlines some criteria around such approvals:
The approver must be a registered user in order for the approval (or rejection) to be processed
If the gate or manual task approver list was defined using a group, any of the names in that group will be possible approvers receiving the email and can click on the provided link to approve or reject. If the user is not yet registered, this action will automatically register that user into the user list.(See (3) )
If adding this user to the registered list would cause the number of users to surpass the assigned count in the license, then this approval/rejection will not be processed. You will need to find someone already registered to approve this request, or you will need to work with your Administrator to get this user a registered account, after which you can ask this “blocked” user to try again.