From the website, http://processing.org
Processing is a flexible software sketchbook and a language for learning how to code within the context of the visual arts. Since 2001, Processing has promoted software literacy within the visual arts and visual literacy within technology. There are tens of thousands of students, artists, designers, researchers, and hobbyists who use Processing for learning and prototyping.
You can download the Processing application for your computer’s operating system (macOS, Windows, Linux). At the time of this writing, 3.5.4 is the current stable version; we’ll use that.