Synergy is an exclusive sales metric that you will only find in Sales & Orders and only for Google Shopping campaigns. Synergy Sales are indicative of a unique attribution model wherein product ads interact with each other to generate more conversions.
Synergy falls into three types:
Direct: When this product ad was clicked on and this same product was purchased in your store. You'll see that the Shopping Ad and Product Purchased match.
Indirect: For a given product, when another product ad was clicked on but it sold this product that you are viewing now. The Shopping Ad shows the ID for the clicked product while the Product Purchased will match the one you are currently viewing.
Assisted: When this product ad was clicked on but led to the sale of another product such as a variant. You will see the Shopping Ad (this product) that was clicked on, and then the actual product that was purchased as a result.