wave() slides each line of the picture sideways by a moving wave: heat over sand, a room under water, a dream. It moves whole game rows, the way the consoles did it, by changing the scroll of the next line while the screen was being drawn. A smooth version would look like a modern shader; this one looks like the hardware.
amplitude is how far a line moves in game pixels, frequency how many waves fit on the screen, speed how fast they travel and axis whether it moves lines (0) or columns (1).
↑ ↓ for the amplitude, ← → for the number of waves, A to change axis and Space to switch it off.