I was following a youtube tutorial, in which a CSS hover effect was used so that when you bring you cursor on the navbar items, a line emerged gradually from the center. How does the whole :hover::after thing work?

This is the CSS code they showed in the video :

```
  nav{
  display: flex;
  padding: 2% 6%;
  justify-content: space-between;
  align-items: center;
  position: sticky;
}

nav img{
  width: 150px;
}

.nav-links{
  flex: 1;
  text-align: right;
  gap: 2rem;
}

.nav-links ul li{
  list-style: none;
  display: inline-block;
  padding: 8px 12px;
  position: relative;
}

.nav-links ul li a{
  color: #ffffff;
  font-size: 13px;
  text-decoration: none;
}

.nav-links ul li a::after{
  content: '';
  width: 0%;
  height: 2px;
  background: #f44336;
  display: block;
  margin: auto;
  transition: 0.5s;
}

.nav-links ul li a:hover::after{
  width: 100%;
}```

```