It is little rectangle popup to notify shoppers that someone has purchased an item of your store. 

If you enable "Notification Pop up" it will appear on left bottom of you store.

Customize popup settings as per your requirements.

Get the notification popup settings in Online Store -> Customize -> Theme Settings -> Notification Pop up

You can change the message to display in popup.

As well as you can change the collection, of which you want to display products in popup.