If you are assigned to another user as their "approver", this means that before they can send a proposal to a client, they have to request your approval first. You can either approve the proposal or deny it and send them some helpful feedback.
In your account, you'll see a new tab on the Proposals list for "Pending", which is anything that has been sent to you to review, has been approved or denied. You can manage approvals from here.
When an approval request is sent for your attention, you'll receive an email notification with a link to Pending Proposals. Simply click on the proposal from this list and it will open the proposal for you to review.
How to deny a proposal
Click the 'Deny' button in the top right, and this will open a sidebar. You can add some feedback here to explain why you're denying it and then confirm. This will email the other user with your feedback and they won't be able to send the proposal until they have fixed it and re-submitted it to you for review.
How to approve the proposal
Nice and simple - click the 'Approve' button in the top right and then confirm that you want to approve the proposal and notify the other user.
Once a proposal has been approved, it can be sent. Once the proposal has been sent, any further edits won't need to be approved.