When you send a prospect an email, you want to make sure they've seen your email and have replied in a timely fashion. The Nudge a Prospect assists remind you to reach out to a prospect if you haven't heard back from them. You'll get a Nudge a Prospect assist if:
You haven't received a reply to an email you sent 72 hours ago.
The deal is not closed.
The prospect isn't Out of Office.
This assist is automatically dismissed if you send an email to the prospect, or if the prospect responds. The assist has a link to write an email, which includes suggested text, so you can easily follow up.