The kyu level of a kata is rated when it is still in the beta stage, so it is the community in general who decides (not the kata author). Though sometimes there can be under-ranked or over-ranked katas and one's perspective on the kyu system may change as their proficiency grows ~~

"dan" is considered mastery-level, and the numbers represent distance from "dan."

So, 8kyu is the easiest, going up to 1kyu. Then 1dan is harder than that, and 2dan is harder than 1dan, and so on.