jj is the name of the CLI for Jujutsu. Jujutsu is a DVCS, or "distributed version control system." You may be familiar with other DVCSes, such as git, and this tutorial assumes you're coming to jj from git.

So why should you care about jj? Well, it has a property that's pretty rare in the world of programming: it is both simpler and easier than git, but at the same time, it is more powerful. This is a pretty huge claim! We're often taught, correctly, that there exist tradeoffs when we make choices. And "powerful but complex" is a very common tradeoff. That power has been worth it, and so people flocked to git over its predecessors.

What jj manages to do is create a DVCS that takes the best of git, the best of Mercurial (hg), and synthesize that into something new, yet strangely familiar. In doing so, it's managed to have a smaller number of essential tools, but also make them more powerful, because they work together in a cleaner way. Furthermore, more advanced jj usage can give you additional powerful tools in your VCS sandbox that are very difficult with git.

I know that sounds like a huge claim, but I believe that the rest of this tutorial will show you why.

There's one other reason you should be interested in giving jj a try: it has a git compatible backend, and so you can use jj on your own, without requiring anyone else you're working with to convert too. This means that there's no real downside to giving it a shot; if it's not for you, you're not giving up all of the history you wrote with it, and can go right back to git with no issues.