The yarn build command

The function of this command is that it bundles the app into static files for production.

Running build takes the application and turns it into something more production-ready. What does that mean? Well, if you're already pretty comfortable with what tools such as webpack and brunch do in terms of turning them into production sites, you basically already know what this accomplishes. If, on the other hand, this all sounds incredibly confusing to you, I'm going to take a little bit of time and explain it in slightly less vague terms.

Essentially, most browsers can't handle code written for Create React App projects just by default. There is a lot of work that needs to be done in taking the code and translating it into something that makes more sense for the browsers, ensuring that it doesn't need to rely on help to interpret everything. From there, the code is also minified! It shrinks things down by renaming functions and variables, removing white space where it can, and doing small optimizations here and there until the code is reduced to a very clean and usable version. Everything is compressed and the file is condensed down as much as it possibly can to reduce the download time (which is important if you're targeting a mobile audience that may not have great internet speed).