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);
}
}