A compact JavaScript frontend framework.
When you want to introduce it into a company, you can tell the suits that it stands for "Business Support" framework.
If you are a nerd, I'm rather going to tell you it stands for "BullSh*t" framework (since I'd expect you to find that more interesting).
But these are only examples of what can be done with it, since the name is simple and flexible just like the software itself. So feel free to make up your own meanings, depending on your needs - just like you can create plugins for whatever you desire.
You don't "install" it. Copy it into your project directory and work with it. If you're using it, it's part of your project.