Amazon SEO means optimizing your product list and makes it appear at the top of the Amazon search result which will lead to better ranking and more visibility and sales.
Amazon enters a keyword to find what people are looking for and just like google; users mainly click on the first few results. If you are selling on Amazon, It means that ranking on Amazon is the most important factor for your successful business.
The higher you rank the more you sell. Once people find your product and consider it relevant, they will probably click and buy it. The more people click and buy your product, it will get generous ranking. The higher your product gets ranked, the more people will buy it.
So basically it is optimizing your product and makes it appear at the top of the search result.