The text in the input box describes a function that determines the color of each pixel. X and Y are the pixel's coordinates and serve as inputs to the function.
Time is also an input to the function, and is what causes the shader to change over time.
Geometric functions are used with the time value to make changes cycle. Without them, most shaders will eventually turn white as the R, G, and B values approach 255.
Ctrl + Enter to recompile the shader. To toggle this sidebar, press space. Use the button to see how these values are used to create different patterns. :)