Destination products are the products that will be shown in the slider widget (in the product page, or the cart page) when the customer is visiting one of the source products, or has one of the source products in their cart. Please refer this What is Product Mapping? for a detailed explanation.
Select "Destination products" from the "Where to search" dropdown. Start typing the product name in the search box to filter mappings that have the product in their destination products:
Start typing the product name in the search box to have the results appear below it:
Then click the desired product to add it to the list of products on the right:
You can either add a bunch of products by a filter, or add a dynamic group of products. Dynamic group means that we will update every 24 hours. We will add new products and remove old ones. This is done by the "Dynamic grouping" checkbox.
This is done with the "Add by filter" button. You can filter using 5 different filters, and add another filter on top of that that will another layer of filtering using the "Add another filter" checkbox.
The destination products are shown on the slider widget. You can order them so the first ones (the ones on the top) are shown first:
You might want only a subset of variants to show on the slider. Click the "Variants" button to have this popup. Click "Done" to finalize your selection: