A millisecond is 1/1000 of a second. Processing keeps track of the number of milliseconds a program has run. By modifying this number with the modulo(%) operator, different patterns in time are created.
Original Processing.org Example: Milliseconds
// All Examples Written by Casey Reas and Ben Fry // unless otherwise stated. float scale; void setup() { size(200, 200); noStroke(); scale = width/10; } void draw() { for(int i=0; i<scale; i++) { colorMode(RGB, (i+1) * scale * 10); fill(millis()%((i+1) * scale * 10) ); rect(i*scale, 0, scale, height); } }