It's a simplified variant of Java really but with a slant for creating graphical applications.

My take on it is that it's good for teaching kids how to code, or for giving non-coders in general a taster of what it's like to program. We even used it at work once to prototype something before building it 'for real' in C++. But if you already know how to code and are looking for a next language to learn for career-building purposes then stick to more mainstream, commercially applicable languages like Java, C#, Python for example.

If programming is more of a casual interest of yours and you enjoy that instant result, instant gratification loop (that you very often get from web dev) then Processing might be just the sort of thing you want to try!