ACOS = PPC Costs / PPC Revenue
Example:
Your sponsored product has had a total of 2$ costs for all of its clicks, and made a total of 10$.
--> ACOS = 2 / 10 = 20%
Understanding ACOS
The following articles can help obtain a better understanding as to why the ACOS is a central metric to measure the performance of your Sponsored Products, as well as how you can define what a "good" ACOS is:
What's a Good ACoS for My Amazon Ads & how can I influence it? (click here)
Why ACoS Can Be Misleading (click here) and why you shout not rely on the ACoS alone (Blog by Amazon, click here)