App Inventor is an open-source project, which means you can download it to your computer. However, in order to run it on your computer, you need to set up a local server. This can be a complicated process.
The Technovation team does not provide support for technical issues you may encounter trying to do this, but there are plenty of good sources and videos online that may help you. If you are interesting in trying it, here's a good place to get started: How to build App Inventor from the MIT sources
The easiest way to to use App Inventor is through a web browser and with internet access. If you need to work offline on your code, try planning things out on paper by making a paper prototype or a flowchart!