The phrase comes from automation and control systems, where a person has to confirm a step before the machine continues. In AI tools it marks the difference between suggesting and acting.
Why it matters
A person in the loop catches the mistakes AI still makes: a wrong fact, a misread question, a tone that does not fit an upset customer. It also keeps responsibility clear. The reply went out because someone at the business decided it was right.
The cost is time, so the review has to be quick. That is easier when the draft shows what it relied on and when most drafts need little or no change. Many teams keep the human step for everything at first and loosen it only for kinds of email where the drafts have proven reliable, which is where auto-send comes in.
An example
An AI tool drafts a reply to a customer asking for a refund. The person on the inbox sees the draft promises the refund today, knows it takes three working days, fixes one sentence and sends it.
In ReplyPen: Every reply is a Draft that a person reads and sends. More in draft or auto-send.