This post is meant to be read in the context of its parent post "Lessons Learned on a React-Native Project". If you've stumbled on this particular page randomly, I'd recommend jumping over to that post and starting from there.
This should be obvious by now, but if you've already got experience in the native platforms, you won't need to deal with the learning curves of several new ecosystems. If you're a .NET (or even any non native mobile) developer, you want to make sure you brush up on what I've talked about in this post, before having a proper crack at React-Native.
The only thing you'll probably need to spend some decent time with is React itself, and thankfully the doco is linked here. :)