Like a flip book, animation on screen is created by drawing an image, then drawing a slightly different image, then another, and so on. The illusion of fluid motion is created by persistence of vision. When a set of similar images is presented at a fast enough rate, our brains translate these images into motion.
To create smooth motion, Processing tries to run the code inside
draw() at 60 frames each second. A frame is one trip through the
draw() and the frame rate is how many frames are drawn each second. Therefore, a program that draws 60 frames each second means the program runs the entire code inside
draw() 60 times each second.
To confirm the frame rate, run this program and watch ...