Watchman is an open source tool created by Facebook (https://facebook.github.io/watchman/). React Native's packager uses Watchman to recursively watch for changes to our source code files across one or more directory trees. Once it detects a change, it automatically rebuilds the JavaScript bundle. This allows us to sidestep one of the slowest and most painful parts of native development.
Much like several of our other tools, once Watchman is installed, you won't have to worry about it. The React Native Package Manager handles running Watchman for us.
Unlike the other tools mentioned, Flow is entirely optional. Flow is yet another open source tool created by the Facebook team (http://flowtype.org/). It's used to add type annotations to our JavaScript code. JavaScript, as you likely already know, is a dynamically typed language. This means you never need to declare a variable as an int or a string. You just declare a variable and set a value. The type is implicitly set based on...