I dont understand what nav a:visited code does to make the links turn white. Can someone explain it?

He combines two things in one, and uses something you have not seen before in this video. Here is how it works.

nav a, nav a:visited {
  color: #fff;
}

is the equivalent of

nav a {
  color: #fff;
}
nav a:visited {
  color: #fff;
}

You can change the color value of either in the second one to help clarify what is happening. Your browser has a default style for visited links, usually a darker purple color. We are overriding that with the a:visited, but only for the anchors within the nav tags.

Let me know if that doesn't clear it up enough.

The visited pseudo-class in CSS is triggered when you have visited a link. If, in your CSS, you set that element to white, it will turn white after you have, in fact, clicked on it, and visited the page.