A reviewer's option in a workflow step. Clicking 'Approve' advances the workflow.
In a Content Flow workflow, the 'Approve' button indicates your approval of a draft page’s content as part of a workflow step. Approval generally means that you, as one of the reviewers, have read the page and have accepted its readiness for moving to the next step.
After your approval, if no other user (or group) in the workflow step needs to approve, then the workflow advances to the next step, or completes if there is no next step.