When an order comes in from Shopify, each line item references an article (product/SKU). If that article doesn't exist yet in Fortnox, Junipeer can create it automatically using minimum data — meaning just enough to make the order line valid (article number, name, price). It won't be a fully enriched product record.
The alternative (unchecked) would be to let the order fail if an article is missing, forcing a manual fix first. Most merchants want this on to avoid sync failures.