var speed= 3;

​

function setup() {

  createCanvas(400, 400);

  rectMode(CENTER);

}

​

function draw() {

  background(0);

  stroke(255);

  strokeWeight(5);

  noFill();

  ellipse(x, 200, 100,100);

  if(x > width || x<0) {

    speed= speed * -1;

  }

  x= x + speed;

  //I don't understand this logic. Check back

​

​

}