It can used as a verb, to indicate the process of approving content. Often, it's a noun, referring to the action of a single person ("that content needs an approval before it can be published").
Approvals are usually serial approvals, where Person A approves, which queues the content for approval by Person B, etc. Less common is a parallel approval where content is submitted to a group of approvers and considered "approved" when one of them (or other arbirary threshold) approves.
The final approval will usually result in the publication of that content.
Approvals can often be effected via permissions. A user might have only "edit" permission, but not "publish" permission. When edits are complete, the content can only be published by someone with the necessary permission.