Could someone please explain in layman’s terms the difference between Automatic, Manual and negative keyword marketing. 
Automatic means Amazon looks at the product title, description and keywords to determine which words will be matched to a customer search when using advertising.
Manual means you specify they keywords that will be matched to a customer search when using advertising.
Negative means a keyword that would exclude your advertising from that customer’s search (e.g. if you were selling controllers for a PS4 you could include “PS4” and “controller” as keywords and “console” as a negative keyword. Someone searching “PS4 console” wouldn’t see your advert).
It’s usually best to start off with Automatic, then when you’ve got some history of what search terms are being used you can use that data to run a Manual campaign.