Cross Sell popup is a popup when you try to add to cart a product and if your product has cross sell product set in it then it will recommend those products on this popup with a nice interface.
Select some products that you want to show in cross sell #
You will find an option that called link products in woocommerce tab panel. Just select some products it will now show on the cross sell popup.
When you now try to add that product to your cart then it show the popup like this. Which is not by default in woocommerce but are giving your customer to grow your sell.