Obviously a great knowledge about classes is far more important than anything prior learning React. Read about them here.
Higher order functions
All methods you use in React are sort of based on higher order function concept like map method. Learn more on the in this post.
The spread operator as well is a very useful feature of React prerequisites and it is essential to learn it complete and use it with care especially when copying an object or array. Read about them here.
Map and reduce and other array methods
Although map is not ES6 feature but it is very well could be combined with ES6 features and terminologies. It is very essential as a one of the React prerequisites and should be learnt perfectly especially in making lists in React. Read about map in this post and reduce in this post and more practice also in other array methods in this post.
Import and export
Of course one of React prerequisites that you always use and need to take care of in all your applications. Read about them here.
Although they are the last to mention but they are not least important one of React prerequisites. They are far more useful than you can think so learn more about them in this post.