Fragment Playground is a live GLSL shader editor. You write code and the picture updates as you type, and you can share the result by link or export frames and video.
A fragment shader is a small piece of code that runs once for every pixel, on the GPU, and returns a colour. You get time, canvas size and mouse position as inputs, so a few lines of sine waves can become something that moves. The link carries your source code in the URL, so there is no account or saving step.
It is a quick way to get a feel for how shader maths behaves.