There are lots of metrics to consider when it comes to Amazon PPC Advertising. Here's a quick reference list to get you started:
Spend: The amount of money spent on clicks on your ads during the time frame selected
Sales: The product sales from orders within 14 days of a click on an ad if that click was made during the time frame selected.
ACoS (Advertising Cost of Sales): Spend divided by Sales. You can look at this as how much you have to spend on ads per sale.
Impressions: The number of times Amazon shoppers saw your ad during the time frame selected, whether they clicked or not.
Clicks: How many times users clicked on your ads during the time frame selected
CTR (Click Through Rate): The number of clicks divided by the number of impressions.
Orders: The number of orders made by users after clicking your ad. Orders are counted if purchase is made within 14 days of the ad click.
Conversion Rate: Orders divided by Clicks
For a more comprehensive Glossary of Terms (for Amazon Advertising as well as for digital advertising overall) you can view the Amazon Advertising Glossary of Terms.