Social posts are one of the four things a client can approve. The full guide is Client approvals; this page covers what's special about social posts.
Turn it on
In Client → Settings → Client approvals, turn on Social posts need the client's approval. The Settings of Content → Social shows whether it's on, with a link there. Business Profile posts in Content → Social follow the Google Business Profile switch instead, because they go to the client's Google listing.
Send posts
On a post, click Send for approval. The post shows Waiting for client until they answer. While the switch is on, a post can't be marked as posted (or sent with Post to Google) until it's approved, and a post waiting for the client can't be scheduled either. Approve for the client lets you go ahead without them.
What the client sees
In their portal's Approvals tab, each post shows its network, its text as it will be posted (with the link and hashtags), its picture and its planned date. They can Approve, Decline or Ask for edits (with a note). You get a notification in the bell that opens the post, and the post shows their answer and note.
After a decline or an edit request
Edit the post, then click Send again. Editing a post the client already approved clears their approval, so send it again if you want them to see the new version.
Previewing as the client
With View as Client, you see the Approvals tab as the client does, but the buttons don't act there. Approving is the client's own answer.