Processing is a programming language used mainly for animations and graphical applications. If your program is written in pure Processing, the same code can be run in many tools that use their own language: Windows with Java applets, HTML pages with Javascript, Android applications, etc. Running an application is done by choosing a mode from the Processing GUI tool.
Here’s a screen shot of a Processing Window containing a program to draw a regular polygon:
Now, to run the program , push the “Play” button, or choose Sketch->Run, and you’ll see the result:

Now, to run this on Android, I commented out the call to the function ‘size’, following is the result on in an emulator window:

You can add to your programs command specific to the mode in which you run. For example you can add an ‘alert’ command in Javascript mode. Better do it in separate files.
You can download Processing here.
To learn more about pure Processing function go to the function reference.