When a visitor is trying to leave your store and moving a mouse cursor to close your store, then a pop-up window with a special offer is shown. Or when a user is visiting your store for the first time, then they get a special offer in a pop-up window. You can set both offers for your visitors or just one of them.
The offers are fully customizable on the admin panel of the app. You can offer a coupon, free shipping, a special price for some product, a gift (the offer with the gift can be created in Unlimited Upsell app) or any informational message to your clients. Ways of how to use the app are very diverse. For example you can inform your visitors about some promotion, about support hours, etc.