This rule is usually used to style headers. It creates a line below the content in the cell. It's a quick way to style your headers to differentiate them from the rest of the table.
When to use it
You should use the Header underline rule when you want to separate your headers from the rest of your content and when you want this look to be consistent and professional.
What can you change
Line style
You can update the color, width, and thickness of the style. The width can either be set as a fraction of the cell width or to Automatic which makes the line as wide as the content in the cell.
Font
Update the style of your font. See the article about the Character style rule for more information.