What does "kettle cooked" mean? "Kettle" is the thing with a spout you boil water in, as far as I know. You don't boil potatoes to get potato chips, do you, though?
While teakettle is the most common association with kettle, a kettle can be any metal container in which food or liquid is heated. It's a less common word for pot. There are also industrial kettles, which are essentially large vats.
Here is a link to an engineeringworld article about how potato chips are made. The distinction between kettle-cooked and modern standard chips seems to be that kettle-cooked chips are fried in batches in the kettle, while modern standard chips go through a "continuous" fry process on a conveyor belt.