Render loop
Code that draws a frame, then asks the browser for the next one, about 60 times a second.
See it in Cellular automata. Press Play to start the loop, or Step to run it one frame at a time.
In plain words
Anything that moves on a web page is drawn again and again. Each time round, the code updates the state, such as where every particle is. Then it draws the new picture. Then it asks the browser to call it again when the screen is ready for the next frame. That circle is the render loop, and it usually runs about 60 times a second.