When a product sells out, most shoppers just leave and don't come back. Now your store can catch them instead. Any sold-out product page shows a Email me when this is back in stock box, so the person who wanted it gets on a list instead of walking away.

It works for anyone, signed in or not. Logged-in customers get their email filled in already, guests just type theirs. If they come back to the page later, they see they're already on the list, with a Cancel link in case they change their mind.

When the item comes back, your store emails everyone waiting automatically. A couple of rules keep it from getting spammy:
Requests are tied to the product itself, so they survive your weekly menu rebuild and fire whenever it truly returns, bundles and kits included.
You get a report too. Under Reports → Back In Stock Requests you can see who's waiting, grouped by product so real demand jumps out, filter pending versus already notified, and flip the whole feature on or off right from that page.

It's off by default. Turn it on from that report page whenever you're ready, and your sold-out pages start collecting would-be buyers instead of losing them.