A CSS reference!

The `::-webkit-progress-bar` pseudo-element is a webkit-specific (i.e., Chrome, Safari) way to style the progress bar of an `<input type="range">` element.

Here's a breakdown of what this pseudo-element does:

1. **Progress Bar**: It targets the entire progress bar area of the range input.
2. **Webkit-specific**: The `::-webkit-progress-bar` prefix is specific to webkit-based browsers (Chrome, Safari) and will not work in other browsers like Firefox or Edge.

When applied to an `<input type="range">`, you can use this pseudo-element to customize the appearance of the progress bar, such as its background color, height, padding, border, and more.

Example:
```css
/* Style the entire progress bar area */
::-webkit-progress-bar {
  background-color: #ccc; /* Set a light gray background color */
}
```
Keep in mind that this pseudo-element is not supported in other browsers. If you need cross-browser compatibility, consider using alternative solutions or prefixing your styles for non-webkit browsers.

Are you looking to style range inputs? I'm here to help!